Personally my favorite enemy was Frank Horrigan (as you may infer from my picture) but I also really liked Dornan because he was really memorable. There are a lot more enemies from the series and I'd like to know which one is your guys' favorite. Also post which game you think had the most memorable villains. By villains I also mean unique enemies because that just makes the topic more interesting.

Frank Horrigan was great and well done, also deathclaw from F1 was memorable because this mystery around missing caravans and super mutants,and this first deathclaw was nice surprise,and also unique, but my most favorite villain from Fallout's is Master, and Super Mutant lieutenant and joke about calling  him by other mutants as Lou Tenant.

Caesar.

Just the easiest guy I've ever found in a game to enjoyably hate.He's a slaver, he kills anyone who doesn't kiss his feet, acts like he controls the player (Least out the few times I've ever spoken with him...), and tries to be above all of his own followers, not with them.

"...What an asshole." ~NCR Trooper

I have to Caesar as well.. He's a power hungry megalomaniac. Personally I like planting a few demo charges on his cronies and then finishing him off with a knife...

Ulysses, I thought the story with him, and how the Courier has been causing huge problems in Ulysses life through indirect actions (unbeknownst to the courier of course) even to the point that the Courier is Ulysses antagonist rather than vice versa, to be very interesting.

Frank Horrigan! Semper Fi! I liked how you could casually meet him while traveling in Fallout 2. Frank Horrigan and those god damn turrets. Best enemy ever.

I'd say Frank Horrigan aswell. The bossfight with him was a really nice touch with all the options to gain support(the turrets,the fleeing enclave soldiers, all of that can be used to your advantage).It was just glorious.