Firstly, the absolutely biggest flaw in the game is that with 4 races, each with its own 3 class options (might, magic and hybrid), the entire Dark school of magic is only available to a single class - the Freemage (human magic). This in itself is unfortunate, but the Dark magic school contains the spell which is the only way of detecting secret doors and such (until at least mid-late game), and some of these secret doors are necessary for completion of various quests, let alone various magic goodies. Rightly or wrongly, a Freemage has to be in your party unless you're playing with the specific intention of not detecting any secret doors until late game.

Second, the Bladedancer (elf might). Dual swords is the best melee damage in the game by a large margin, despite an elf not typically being considered a “tank” class.

The other two characters offer more flexibility. Back to magic, the Freemage can GM Dark, Air and Prime, and M the rest. That's not too bad on the surface, but unlike previous MM games you won't get enough skill points before beating the game to max out even the skill options you do have. So since you'll be focussing on the ones she can GM, you want somebody else that can GM the others. There is no character that can GM every other school (Light, Earth, Fire & Water), but there is one that can do 3 of those 4: the Rune Priest (dwarf magic) can GM Light, Earth and Fire. Light has most of the healing and many of the useful protection spells, Earth is crucial for the Regeneration spell and poison, and Fire has a good mix of protection, utility and damage. Water has one of the most useful spells in the game (Liquid Membrane), but you don’t need GM for it, and the GM Water spell seems to be the most common GM spell to find on a scroll – and you won’t really miss it if you don’t use it. The Freemage is more than enough for M Air, and the Rune Priest is therefore a clear choice for your 3rd character based on the first 2.

Your last character probably has the most flexibility. If you favour a might-heravy party, another Blade Dancer could work. But for various reasons such as race-specific quests and equipment, the ideal in MMX is when you have each race represented in your party. Which means an orc. Again, if you want a might-heavy party, the Barbarian (orc might) is an ok choice, but he can GM both mace and spear, which leaves one GM skill he won’t be using at the expense of the combat-based secordary skills that the other races can GM. Similarly if you want to go for magic heavy, the Shaman (orc magic) can GM Water, Air & Earth if you really want to max out Water. But with 2 magic characters already, in my opinion the perfect compliment here is the Hunter (orc hybrid). And this is for one reason: once you get promoted, you get access to the Javelin skill, which allows you to throw your spear at an opponent in missile range, and drag them into melee. The ability to pull an opponent spamming you with various buff-removing or other negative-status spells into range of your tanks, without having to walk 2 or 3 squares into meele range while they get another few free shots at you, was one of the most enjoyable things I’ve experienced in any combat-based CRPG. Your Hunter will become your favourite character even though pretty much everyone else can deal more damage or cause more direct woe to your opponents – the ability to drag an annoying opponent into melee range will far outweigh any other maxed-out skill the rest of your party may enjoy.