It looks like the most unbalanced versatile build ever .
Kensai-Mages are powerful and versatile, but they're not the
most versatile class. If you want versatility, try the Swiss Army knife that is the Fighter-Mage-Thief. Fighter-Mage-Cleric is also pretty versatile. Both also become very powerful, especially if you remove the experience cap (i.e. you can reach level 40-40-40 since you're not restricted to 8 million XP in ToB, less in SoA).
As for powerful classes, the reigning champions (judging from others' comments over the years as well as my own experience) seem to be Berserker-Cleric, Kensai-Thief, Kensai-Mage, and Sorceror. Wild Mage is a nice second-tier class, but a bad surge often forces a reload by killing innocents (thus rep and quest issues), killing half your party, nuking your gold, and other horrible things. Monk is also a solid second-tier class IMO. Barbarians are also decent, despite being worse than Berserkers in most ways (can't dual class, rage only adds immunities if you've maxed your STR/CON, no grandmastery, armor restriction IIRC), since their movement speed bonus is damned useful.
If you don't mind wasting most your party slots, and you haven't installed a mod that changes bard song behavior, try a team of PC + 5 Skalds. Bard songs (except Jester) in vanilla BG2 affect everyone on the map, you can hide the Norse Barbershop Quintet in a corner (preferably invisible) and turn your PC into a raging ogre with a disgusting +20 (IIRC) bonus to hit, damage, and I think AC.
Also, get an Inquisitor (e.g. Keldorn) in your party. They have two of the most useful abilities ever: True Sight (once per day per 4 levels or so) and Dispel Magic (once per day per 4 levels or so,
cast at twice their level).
Also also, BG2 Edwin (who doesn't play well with Minsc and IIRC Keldorn, unfortunately) is fantastic. He gets something like +2 spell slots per spell level, in addition to the +1 for being a specialist (Conjurer).
FYI this is all BG2-specific except the FMT/FMC multiclasses. Those can still be useful in BG1.