List what your fighting style is, and with the character that best suits that, I'll start.

I like to play around with my enemy, while building up power and eventually wiping them out. Ultimecia =]

Quick Hit & Runs
Onion Knight suits me for this because of his Chain HP Attacks
Of Course,Tidus is also Good for this with his [Dodge] Type Attacks

I have a real knack for The Emperor and making his traps work. I have made some real chumps out of friends who fancy themselves l33T SEPHIROTH MASTAHS. And nothing is more satisfying than landing Starfall.

Yep I use him alot as well, one of my mains, he's great in Ad-hoc, people practically beat themselves up when stuck in a l;ightning crest

Im always in the air dodging and crap lol
so my mains are of course zidane tidus or squall
mainly squall:D

Have you tried Kuja? Despite his annoyance, he's really good when used right, and is mainly an aerial player, but he can good from an array of different distances =]

As well as Ultimeca

Depends on the character.

Warrior of Light -- I'm usually a bit nervous everytime I try and use his moves since they're so slow, but his shield attacks->Rune Saber is probably one of the most powerful Brv->HP Attacks since it lets you finish the final blow. Since my WoL build is also Defense+EX build, I take care of the fact of not getting critical'd, thus you don't see me often attacking. Unlike many players, I'm used to ground play, so you'll see me using White Fang often. Very often; maybe even too often for a very experienced player like me (but the White Fang is mostly the reason why my shield attacks often hit)
Garland -- Gigant's Axe is so awesome I don't know what to say. I use either FBC build or pure EX Build. Under FBC build, it's so hilarious to see people going "" when they get hit by Diving Axe so hard, even in EX Build. Diving Axe is so awesome I don't know what to say. Under EX Build, people get too afraid to hit me, that's why I go aggressive there.
Firion -- Even though I've used Firion almost 87% of the time I've played the game, I still cannot get over the fact that I'm almost useless in the air. I am often found at the ground, casting the occassional Fire and Thunder. If I get stuck in the Air, OmniAirDash->Swordslash is my best way around, as I've practiced reflexes to be able to dodge chase sequences with human players. My build is the EX->P.Atk build; Firion's attacks are so devastating when they hit, but that's a pretty big 'when'. Fire is the most used move since Shield Bash is love.
Emperor -- First rule is that the Blue Flare should always be present. Then, plant maybe 8 mines around self, then hide in them and launch Starfall. Lol. But in wide places, just keep casting Flare and Dynamite and it should be fine.
Onion Knight -- Almost similar to how I play Warrior of Light, but unlike most Onion Knights, my main Critical Hits go to Riposte. Most people would expect Onion Knights to be aggressive -- mine isn't. And, I'm staying on the ground most of the time too. I can use the magic attacks pretty well, it seems, but that doesn't mean I suck with the physical attacks. In fact, an EX Build with OK (using Cleaver) is so scary it's one of the most powerful -- I can deal 3000 Brave Damage on other Bartz players with just one combo, and around 2000 if I chose to use Guiding Swipe/Swordshower. Who said OK's EX mode wasn't overkill? It clearly is. >_>
Cloud of Darkness -- Hybrid FCB-Wall Rush build, and as expected, CoD Particle Beam Spams, especially 0-Form. Using Tentacle of Suffering-2nd combo nets too much EX Force, and is great for storing EX for my Brv; not to mention that Dodge Cancel->0-Form cancel is almost a sure hit.
Cecil -- Paladin Arts is so cool, lol. In any case, Cecil is probably the character I'm so chicken with. If you play my Cecil, you'll often find it dodging like spikes are coming out of everywhere, running away when possible. Build is hybrid EX-Wall Rush build (Earthbreaker), and I usually stay near the ground to abuse the Paladin attacks a lot better.
Golbez -- I either use the EX Build or the FCB Build, but the strategy stays the same. In the attempt to mix combos, I've found that *insert name of the 3-laser firing balls attack here* is such a useful setup move. Not to mention that some of his attacks are the swiftest to execute, and is the only Chaos character with an easily branching combo. The main difference of my builds is that in EX Build, I actually go more cautious in order to use Black Fang.
Bartz -- I have so much fun using Bartz. With his variety of attacks, I can keep switching attack patterns from ground-control to permanent-air to defensive to whatever you call it. Build is FCB Build (Heaven's Cloud) since I don't know what else to do with him -- it's hard to think up of a good set for him since he's above average at many, superior at none.
Exdeath -- the epitome of what I call ownage. My main form of attack is Delta Attack and Reverse Polarity, but my best attacks are obviously Omni Block and High Block. My Exdeath hasn't lost to any human characters yet, I am serious; Exdeath is so BLARGH that I laugh at the many EX-Cloud users with a well-timed Omni Block, as they take an Almagets right to the face. Build used is standard Block Brv Boost (Enuo's Scourge+Snowpetal), while increasing defense too. But Warrior of Light is still the sturdiest of all my characters, but Exdeath boosts his Brv by 500+ everytime I get a block... :3
Terra -- I only recently started using her, and I must say that HER EX MODE IS TOO IMBA. I mean, wth? The main problem with using Terra is maintaining the EX Mode for long, this is why I prefer using Nirvana Staff instead of Stardust Rod. Build is standard EX-Magic Attack build, emphasizing longevity of EX Mode through Nirvana Staff.
Kefka -- Another character where I said "IMBA EX MODE", just go into EX Mode, cast Waggle Wobbly Firaga, go into opponent's face and cast Scattery Split Blizzaga, and see the opponent's Brave eaten faster than you actually realize it would. Build is standard EX-Magic Attack build, emphasis on M.Atk (Stardust Rod)
Cloud -- EX-Wall Rush build, obviously, but my favorite attacks for cloud are his two rising-diving attacks; they're very effective against human players. Slashing Blow for finisher once EX Mode is on. I also have to make sure that Cloud's HP remains at 100% since he's one of the most imbalanced characters in EX Mode.
Sephiroth -- BLACK MATERIA FROM AFAR HELL YEAH. Uh, yeah, I'm actually using his spells more often than not. I play Sephiroth like I would play a slower Cecil, but with more brute force. I use an FCB-P.Atk Build (Ragnarok), nothing much to say.
Squall -- Standard EX-P.Atk build (Cleaver) = a pretty strong Squall. I never realized how BOOM his attacks go in EX Mode, that's why I started using him like hell. Coupled with the skills in dodge cancelling, Squall is probably the character I use with most aggression.
Ultimecia -- I just have no idea how to make her fighting style exciting. In any case, I use standard FCB->Brv Boost build for Ultimecia, emphasis on intake range. Regarding battle strategy, how I play Cecil and Exdeath mostly (yes, that means I'm very non-attacking with Ultimecia)
Zidane -- FCB->Brv Boost Dodge build (Ozma's+Adamant Chains), abusing Free Energy is so fun. With a lot of bravery stacking up, I play Zidane as a Golbez-Cloud of Darkness hybrid, as would be expected.
Kuja -- ...this is the only character that I honestly don't use because no matter how much I try him, I find him too boring to use.
Tidus -- EX-Brv Boost Dodge-P.Atk build, emphasis on getting the dodges right (World Champion + Adamant Chains). I play him more like a dodging Exdeath instead of the usual aggression, and when you play my Tidus, you'll usually find me shooting balls at you more often.
Jecht -- FCB-P.Atk build (Ragnarok), you'll usually find me trying to Jecht Block you all the way until I can batter you with some of the most powerful P.Atks to ever exist in the game. I play Jecht slowly, but when I unleash fury, I UNLEASH it.
Shantotto -- BIO SPAM. ._. I made Shantotto's build the most unique, emphasizing on Brv Boost on anything. My set actually includes both her P.Atks and Stun, and I only have Bio in the ground and Bind in the air. There's nothing much to say about using Shantotto, really.
Gabranth -- First, jump to a secure place. Next, spam EX Charge; if you get caught, Omni Air Dash out of the way. Third, once in EX Mode, unleash hell. Actually I emphasize on Aeroga since it's such an awesome move. Build is standard EX Absorb-EX Longevity with a bit of P.Atk blended in to it.

That's pretty much it.

Bartz is my new main.

Anyone who thinks he can't chain a hp attack obviously don't know about the close to ground 2 hit Slidehazzard-Soul Eater, the same 2 hit Slidehazzard close to the ground-Dodge Cancel-Wind Shear, Reel Impulse-Soul Eater and all of the above with Goblin Punch.

All his ground moves save flood combo from Reel Impulse. And then there is the frankly almighty Reel Impulse-Forward Dodge-2 hit Slidehazzard-Dodged Cancel-Goblin Punch.

Oh, and Slidehazzard-Paladin Force. Hard to do though.

Add in his awesome brave game and his ability giving moves and you have a winner

Now my style with him is to interupt people in the middle of moves with Holy-Flare or Reel Impulse combos. Add in the the odd bit of Flood pressure and dodge attacking and errm, you have me =]

My style is probably the newbie hit-them-hard technique lol..
Im trying to get better at Dissidia tho, so it might change.
But I do love Blizzara'n people with Terra then Tornado spamming. It worked on Chaos, itll work on others lol.