I'd like to share my thought on the matter: which was Hulk's greatest enemy? Sure many of you may be thinking about a lot of names, but my answer to that one is quite different, the bigger threat Hulk has ever faced was in the hands of his own creators. 
I'll explain: it was not that big a surprise when his own comic got canceled at his sixth issue (he only got a full comic of his own again years later), I mean, c'mon why couldn't Marvel just have 1 idea and stick with it? In each issue we could see different aproaches on him: Banner changing to Hulk at night, Hulk controlled by Rick Jones by means of telephaty, Banner changing to Hulk using a machine any time he wanted, etc etc. This was very bad for old Hulk, at first they had no real clue of what to do with such an awesome character and just tossed any ideas they had... =/
(sorry for possibly english mistakes, not my primary language)

" @Veidt: 
All True. But then there was " Tales To Astonis h"Hulk's greatest enemy has always been Bruce Banner.BTW. "

I like Red Hulk.

 
Perhaps a better way to say it is that Hulks greatest enemy was his unexpected popularity. At the time, Stan Lee was probably throwing out any idea he had to see what would stick with the readers. The Hulk obviously did, but had no long term story arc for him. Thus whenever a new writer took over, he gave not only his own slant to the Hulk, but explored another avenue of the connection between Banner and the Hulk and why. And despite the many variations, the Hulk was such a way in that they all made sense and worked in thier own way.
 
What you consider the Hulks greatest enemy could also be seen as his greatest advantage. Almost every other superhero is static. In that his motivation, powers and character are always the same. The Hulk's changes, adding a twist every so often. Instead of always being smart Banner, dumb Hulk, the writers added the twist of smart Hulk as well. Instead of Banner/Hulk, they were seperate entities. Instead of having to always work with during daylight Banner, night Hulk. They made it that anger transformed Banner.
 
The Hulk had the advantage of altering what he does and why, keeping him fresh and having the reader and fans wondering what would happen next to him. All the while never taking away the basis of a meek scientist tied to a giant monster. 
 
Of course that is why I enjoy the Hulk. Your milage may vary.

Hulk's enemy is Banner... Psychologically speaking. 
 
Physically I would think it's Abomination, but Thunderbolt Ross's Red persona might've replaced him. 

seriously between Red Hulk and Abomination.

Ross

Ross or Leader

Himself

Bruce Banner. 

I always saw the Leader or General Ross as his greatest antagonist, The Hulk may see Banner as his greatest enemy, but I disagree. Ross has been and always will be one of his greatest antagonists, he is a man obsessed with destroying at any cost because in his world of black and white, The Hulk is a gray area. That and the fact Banner is nailing his little girl certainly fuels his firey hatred of him. The Leader on the other hand is The Hulk complete opposite number. As strong as the Hulk is, The Leader is as intelligent. It's the ultimate battle of brains vs brawn.  

Onslaught is his worst enemy. Onslaught messed with Hulks mind multiple times even made him have an illusion he destroyed the some of the avengers while killing Onslaught. Cable had to help him defeat one of Onslaughts mind control which almost killed the Hulk. Onslaught could of easily fatalily mind zapped hulk killing hulk. Onslaught let Hulk destroy his armor only because he was secrelty growing stronger until he became power itself.

Himself...

Hulk's biggest enemy like in a villain is abomination (I think) but my favorite is the leader. By he way didn't the leader turn in a normal human again. I think the leader would have a back-up plan to prevent this or a way to change him back in the leader.

It's definitely between Abomination, Ross, and the Leader I don't really know who it would be from there though. 

abomination an leader

IMO the greatest enemy of the hulk is someone he cant trade punches with....who even though appeared only in a few issues will always haunt him....Brian Banner.

The greatest enemy of the Hulk is Bruce, and vice versa.