Let's see...
I've never cared for any of the WoD games, but that has little to do with the system (which a friend of mine used to great effect in his own WoD-system based RPG, "American Jihad"), and a lot to do with execution. The way the game is set up, rather than getting "horror roleplaying", you end up with a lot of politicking at best, hack and slash at worst. If I want to play a game about politics, power-mongering and social climbing, I'd much rather play Fading Suns or my own D&D homebrew. That's not to say that the premise of these games is a bad one, it just doesn't work as advertised (and, barring a miracle, probably can't).
I liked WEG Star Wars... I don't really understand what the problem people have with the template system was, given that from what I could see, they really didn't do anything except pre-assign the PC's free 6d worth of stats, make a list of likely skills that the character would have, and give some bonus equipment. Very few of my players ever even bothered with them, since there didn't seem to be any reason for them.
Though, that said, I vastly prefer d20 Star Wars. The action and Force rules are much better... my only peeve is power scaling. The problem with d6 Star Wars was that powers on par with the main characters of the films were completely out of reach for the PC's (except for, perhaps, in a few areas, if a player plunked all of his Character Points into one or two skills). d20 Star Wars suffers from the other extreme- advancement tends to be so quick that PC's can rapidly out-power the characters from the films- and, on top of that, tend to be optimized to the point that the NPC's as written wouldn't stand a chance against the PC's, who typically have piles of custom-designed equipment and higher skill ranks (one Consular 7/ Master 1 in a game that I ran boasted a "move object" score higher than Yoda's... at 8th level. Given how strong she was in all areas-the character had an 18 Intelligence, thus 11 skill points each level- at higher levels the PC would have been easily the most skilled force-user in the galaxy. It can be rather hard to challenge PC's like that for very long...)
Back on topic, however, I'm not sure what the worst RPG system I've ever played was. Maybe Fuzion... didn't impress me one bit. HOL, but that was a feature, not a bug.

A friend in high school designed a "Final Fantasy" version of D&D 2nd edition that was munchkin city, and rather dull... but that doesn't count, given it was homebrew...