Actually, while the 2nd edition rules make the most powerful class a fighter/mage, (snip...)
A multiclass fighter mage elf is decent, but far from the best. Level limits mean that they will never get 9th level spells, for even high attributes can't help multiclassed demi-humans (also, I think elfs can't get to level 17th as an MU even with high int). A dual classed human could be fine, depending on what level you changed, and whether you changed to fighter and M.U.. I mean, getting a stronghold, an army and area spells is a pretty good way to start painting the world map's hexes! But still, if you want versatility, making a M.U./Cleric is probably a better bet. Of course, if we are talking about Arthas, and you feel confident, you could try to go for the dual classed M.U./Psionicist. But even if your GM is playing Dark Sun with the assumption that the PCs always win, and always get more XP, ad infinitum, most groups will probably stop before you can get to 20th level on both classes.
By the way, to understand why different classes in D&D have such different power levels, it is important to understand how the game was envisioned to be played. If you keep playing, week after week, with the same PC, where there is little chance you will ever need to retire, or die, or where dying basically mean you make another P.C. with the same amount of XP, then of course this is going to be broken. But the idea here wasn't that. All the random generation, the requirements for belonging to a class or a race, the different X.P. needed to level ups, the maximum level for demi-humans, the reducing constitution for being ressurrected, all that together created a kind of gamble game. Every PC is basically a gamble the player can make, given his results on the random rolls. A set of rolls might be interesting because it would make a good M.U., for instance. Should you make a human M.U.? Like
Azraed the Level 1 M.U. killer said, they are rather weak, and can easily die. It can be a whole lot of trouble getting them to the point where they are interesting. Maybe if you have several good attributes, you can make an elf. You will never get some of the coolest stuff, like wishes. But it is easier to make your character survive, and he will be a pretty cool character, even if he will never be the best mage around. Maybe you rolled really good strength? Maybe you could then start as a fighter and multicalls to M.U. later, though that takes even more time! Maybe you rolled some very specific stats, and you might be able to try some of the more exotic classes, like the paladin? Each character is a bet, and the bet continues throughout his life, with every adventure, every item you interact, every magic item you collect. A bad encounter with the deck of many things could fuck up your P.C. forever.