I don't mean an avengers-villain like ultron
Iron man has the mandarin 
Spider-man the green goblin
Wolverine sabretooth
moon knight busshman etc
but what does ant-man get? oh and while your here could you sum some hawkeye enemies up too (not bullseye i consider him as a daredevil enemy)

Hank Pym?

I say if it's anyone, it's Ultron

What about Egghead?

Egghead and Ultron both could be considered for this. He is usually the one that comes up with the ideas to bring Ultron down, not to mention he literally made the guy

I thought Ultron was more of an Avengers villain. Pym's never beat him alone.

@gambit987: Him and Absorbing Man seem to have quite a beef (as seen in Avengers Academy), and Creel really seems to hate him.

Egghead - first appeared in Tales To Astonish and had regular run ins with Pym until his death. Probably the closest Pym had to a solo arch-enemy.

Doctor Nemesis - First appeared in the Astonishing Ant-Man Marvel features comics in which he spied on Pym, experimented on Hank & Janet, stole Pym's Ant-Man technology and framed Hank for a bunch of crimes. If writers remember he existed he would probably be Pym's current arch-enemy.

@walker696: this. Whirlwind could be counted I suppose although he seems to have more of a beef with Jan then Hank.

Himself.

I see Ultron as both Pym's and the Avengers' nemesis. It's possible. Just look at Doom, Reed, and the FF.

Pym created Ultron, based on his own brain patterns. He's a dark, twisted Hank, like Venom to Spidey. Ultron even tried to imitate Janet by creating Jocasta. He's Pym's greatest failure and greatest enemy rolled into one. I think the answer is clear.

it used to be Egghead, then he died. Ultron would be top choice since then, but they rarely go 1-on-1.

Ultron, Egghead, and Whirlwind are the most common Pym villains.