Hardest enemies?

Legendary Deathclaw is the toughest NPC in the game.

The ending is somewhat difficult, but still not that challenging. Try fighting the Radscorpion Queens.

Only 2 Radscorpion Queens in the game. Go to the fire house at Camp Searchlight. She has about 5 scorpions (normal size or giant sized) guarding her, so it may be a decent fight.

Multiple Mother Deathclaws... Not really. Deathclaw Promontory is an unmarked location on the map that is on the east side of the river. It has 2 MDCs, an Alpha, and a quite a few normal ones as well as some young Deathclaws.

Dunno about Cazadores. Try finding the Legendary Cazadore, it should put up a decent fight at least.

Actually it's the only add on for this game... The legendary cazadore wasn't hard at all for me. Try turning your game to the hardest difficulty and just either going in to the promontory area, or in to the quarry in between primm and new Vegas. There is a bunch of regular death claws along with alphas and mothers.

After the LDC... there is nothing. Legate is nothing once you have destroyed the LDC. He is yours. THE WORLD IS YOURS!!!

Now that I think about it, Deathclaw Alpha Male and Mother Deathclaw in Quarry Junction are pretty tough. I died a few times even when using Psycho+Med-X and guarding against their lunge attacks on my 10 strength/endurance melee character.

They're designed to be that hard though, so it makes sense. Go there if you haven't already.

Cazadors are possibly my least favorite enemy in the game. They always appear in packs and rape you. And they poison as well.

Actually, they're my second. I still hate Radscorpions.

You want a challenge?

I guess you could try killing the ghost in the dead money add-on, but you can't actually hurt them >.>
Perhaps go through the Deathclaw sanctuary again once all the deathclaw have respawned using only grenades, dynamite, other thrown weapons only.