My favorite systems and some limitations of them.
1> Hero. Hero is one of the orginal 'generic systems. I played Fantasy Hero way back, since we were all so fond of Champions. I even ran a 4th ed FH game for a few years. Despite this, running FH feels a bit like running Fantasy Superheroes. A valid genre perhaps, but not really a Fantasy Game, as most people envision it. The freedom tends to make players ask for unusual races and characters more often, and makes the group a bit more bizarre. I loved my FH game, but it was not a true fantasy game.
I also ran a historical game and tried to use FH. It still didn't work really well, because it's just not what the game was designed to do. The game flourishes when you have the freedom to do the bizarre, and it's strength is the breadth of character concepts avaiable..restrict them and you weaken the game.
They should have just called Hero, Superheroes. That's where I saw it's strongest elements. I played in one Superhero game with this and it was awesome the options that were available. But I tried making some "normal" characters and I just didn't find them that interesting. Without the capability to utilize the abiltiies to shoot fire from your eyes the system kind of fell flat.
But I must say that the system generated one of my favorite characters of all time, SUPERNOVA MAN. He was basically a completely ordinary person who had but one power. The ability to go Super Nova and basically destroy the planet and everything on it...including himself.
4> Deadlands (pre d20)..I loved this game system. The world needed some tweaking, but I loved adding 'cards' to a western game, and the initative systems, and a few other doo dads. Magic was just plane FUN to use. I'd never run Deadlands with another system, and I'd never use the deadlands system for another game. It's perfect for what it does, useless for everything else.
Oh yea, I forgot about Deadlands. My favorite Initiative and Spellcasting systems ever. It was a unique system that A) worked and B) fit incredibly well with the setting.
I didn't like the character creation much, however, as play balance was skewed horribly. Don't get me wrong, making a character was fun, but I saw a big swing in power levels after making a few.