Back in the day I probably played a couple of really bad RPGs I've blotted from my memory. However, the one that stands out as my least favorite is Champions/Hero System, even though I played in a number of Champions games.
Curiously, I like GURPS and dislike Hero, which may strike some people as perverse or even blasphemous.
- GURPS's sweet spot is for mere mortals, no matter how heroic, and they come out reasonably well. Every character in Hero that I've seen, even normals, spies, fantasy characters, and far-future people read more like low-powered superheroes with a theme. A few Murphy's Rules describe how "normal" people in Hero can take unrealistic amounts of punishment, including falling out of a three-story window without significant damage.
- GURPS has only a few stats, and fairly minimal complication (at least after character generation). Every action that requires a dice-roll is essentially a skill roll, plus damage rolls for physical attacks; Hero (last I saw) had one 3d6 roll-under system for skills, and another Nd6 roll-over system for power use.
- Disadvantages are really disadvantages, defined very precisely, and not hand-wavy "tragic flaws" the GM invokes only when he feels like it.
- Advantages, disadvantages, and skills are mostly in plain English, and not some arcane superposition of powers in frameworks, with its own jargon. (Although, to be fair, GURPS 4th Edition added Enhancements and Limitations that almost work like Hero ... but at least they're stated as percentages, not parameters in an odd formula.)
I'm not really a superhero fan, so perhaps that biases me even further against Hero.
Having said that, except for truly pathological rules, having a good GM and a system suited to the type of game he runs often matters more than technical details of the system.
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)