HERO System is the most scalable roleplaying game ever made. It can accurately simulate any genre, any super power, any magic spell, and any equipment. It does this by using a points based system, with modifiers, advantages and disadvantages. Anything can be simulated with a combination of skills, powers, and modifiers.
The basic game system is rather complicated (there are 14 ability scores), but the game uses the same few rules and concepts for everything. It uses the same rules to see if you can hit an opponent, hack a computer, or even to see if your poor old Aunt Bunny has fallen ill this week. Everything is based around skill checks, which are done with a simple 3D6 roll. Effects (when effects are variable), are done with one or more six sided dice.
Each character was represented with Ability Scores, skills, powers, and disadvantages. The power list is very comprehensive, I have yet to think of an effect that cannot be simulated using the powers in the HERO System rulebook (there is a large section of power modifiers, which help out immensely when trying to create that special item or power). The skill list is largely populated with modern day skills, but it has several "roll your own" skills available that lets you learn those skills that no RPG has listed (so you can actually become an expert calculator repairman if you want to). The disadvantage list is also very nicely presented, and has a few generic ones that can be twisted for almost any use (you want your character to have a fetish for pens, or a missing eyeball, well you can do it).
HERO System also managed to fix several common RPG fallacies. First off, armor doesn't make you harder to hit, it actually absorbs damage (like it should), secondly, they separate minor damage (getting the wind knocked out of you), with real damage (broken bones, bleeding and the like), into body and stun. So it is actually possible to knock people out without really injuring them, or kill people without knocking them out first (just like in real life).
Ok, we have now firmly established that the system can do anything (and it really can), but now for the inherent drawbacks in the system. The biggest drawback is that when playing with high powered characters (or Superheroes), you often have to roll an insane amount of dice for combat damage (10D6, 12D6, or even 18D6 or more), this is a real pain to count up. The other drawback is that the 3D6 skill system is very unfriendly when pitting characters of different skill levels against each other. Your warrior can probably beat up every guy in the bar without even being hit, but when he won't even be able to land a punch on the high powered supervillian. Fortunately, there is a simple solution to that problem, just replace the 3D6 with a single D20, that goes a long way towards making the game more balanced (no need to change anything else, just consider a 1-3 to be a 3, and 18-20 to be an 18). Finally the system is really lacking in detailed source material, the Champions superhero world is the only world that is really fleshed out in any detail. The most interesting worlds only have a single supplement (such as the Ninja Hero supplement, which is a work of art, but leaves me wanting more).
This game is best for a game master who is willing to put in the time to create his own universe (or who wants to use the Champions superhero one), there simply are not any killer published game worlds out there waiting to be used (no Greyhawk, Forgotten Realms, or Middle Earth here). I suggest using the 4th edition, as it was the best one (it is out of print now, but check eBay), and had the most source material available (mostly awesome books, but they are all generic, and don't provide the GM with a useable world).
Now just for fun, here are the characters from the group I ran back in high school.
FRED, "The Worlds Smallest Martial Artist", (He was only 6 inches tall and had a strength of zero but he made up for it with mad kung-fu skills).
"The Grey Goblin", he had a shield and an intelligent talking magic sword that would cause him to dimension hop randomly every few months.
"Bomberman", He was one of the most interesting characters ever. He stole his powers from a godlike being. But he never gained full control over them. He could fly, (sometimes). He could turn people into shoes. He had a Logo made out of energy that floated an inch above his chest that said "Bomberman", (which caused lots of trouble since he had no way to hide those big glowing letters. Finally he was almost immune to physical harm, but was highly susceptible to energy based attacks. He died in action taking out all of St. Louis, when his final power was revealed. His final power was a nuclear explosion activated on his death. The Grey Goblin escaped the blast by teleporting to the Jiam Sphere dimension and lived a long life afterwards. But what happened to him is another story.