Fighters. Good old solid, dependable Fighters.
Some people think they're the most boring class, but if that's so, then why are 90% of all mythical, legendary, or modern fictional fantasy heroes warrior types?
Sure, in D&D the Fighter has the fewest special abilities. In a game where the non-spell casters are pretty cookie cutter in abilities and class bonus functions, and Fighters don't really get any, they look boring on paper.
But in play, most of the stars of the show tend to mirror fiction--they're the Fighters. At least in my experience. Even the high level spell casters, who can steal the show, usually do so in one or two encounters, then the Fighters take care of the rest.
Anyway, my houseruled Fighter pretty much looks like the Classic D&D Fighter we all know and love (or hate). As per Mentzer, at high levels they get Multiple Attacks, Parry, Disarm, and Smash maneuvers. I don't want to fiddle with "can I hit it on a 2?" questions (I'm using ascending AC and attack bonuses rather than matrices), so if they get multiple attacks, they just get multiple attacks. I doubt that will make the Nightwalkers or Elemental Rulers any less scary. Especially since I've capped the basic 4 classes at level 20.
The one AD&D rule I borrowed for Fighters is Weapon Specialization. At 4th level, a Hero can choose one weapon they prefer, and get a +1 to hit and +2 to damage. (No silly 3/2 attacks or anything like that though.) I'm considering dropping the level of WS to 3, for the single reason that the level is called Swordsmaster. It would seem thematically to fit, and would actually give the Fighter something besides that extra 1d8 hit points at that level. They already get a bonus to hit and better saving throws at level 4.
In a departure from Mentzer, the chance to become a Paladin/Avenger/Knight no longer exists. Paladin and Avenger became their own classes, then I combined them and brought back the Knight into my own Cavalier class (more on that in the future). Also, with the level cap lowered, I changed around the levels when multiple attacks are gained.
So my Fighter is a little more spiffy than the Classic standard, but not really that much different (and I avoid the need for Weapon Mastery cluttering up my game by giving Fighters only a flat bonus to one weapon they prefer).
Good old Fighters. When the chips are down, the M-U and Cleric are out of spells and the Thief's been cursed by some bit of treasure she was trying to steal, they're gonna pull the group's collective ass out of the fire.