1) Berserker dualled to mage at level 10. More cheesy than Kensai to Mage, because can wear bracers, has more spells (kensai usually dual to 13, berserkers at 9) and can equip helmets. Also being a capable fighter and using simulacrums, each one loaded with his own spells/buffs/summons AND berserker rage, means one small, unstoppable army. Whereas kensai would loose control from an enemy spell/ability.
2) Beastmaster dualled to cleric at level 12. This walking cheat, begins as a beastmaster, which is the ONLY ranger kit which can actually dual. At 12 he gets the 3 animal summonings as low level spells and much later a find familiar skill. He can only use Club and Quarterstaff. By becoming Cleric, despite BM is initially unable to equip metal weapons, he can use and put one mastery point to Warhammers, Flails, Maces, so the entire Cleric weaponry is fully utilized and you do not miss on great weapons (Crom Fayer, Flail of the Ages, Frost Morning Star from Dorn...), and this is kept as is even after old class reactivates. Then as it progresses and develops, ALL druid and cleric spells become available, minus Druid Transformations and HLAs, minus 2 unholy cleric spells, and the Symbol Stun is lost because the divine spell scroll is crammed full (but you can always memorize it 1 level up before it gets out of screen permanently). Chaotic Good is a nice idea, because getting a Fairy Dragon able to use Invisibility Radius 10 spell, is always a great party enhancer.
3) Kensai dualled to thief, at level 13. Kensai gets additional bonuses once for every 3 levels they gain. And other additional bonuses once for every 4 levels they gain. So gaining 12 levels (count out beginning at 1) pretty much rounds the bonuses sum about. Thief HLA "use any item" completely destroys ALL kensai cons; you can use, equip and wear ANYTHING, even class specific items! Let alone the moment you dual to thief, you can use and put 1 mastery point to crossbows, shortbows, darts and slings, thus destroying the no-missile con kensai start with. With Kai and passive bonuses in the picture, as well as initial kensai-career-hitpoints, and overall backstabs, grandmasteries (i myself got grandmastery at katanas, scimitars and 3 starts in dual wield) and even upfront melee combat, this is a walking killing machine.
4) Raw avengers. Extra mage spells, shady combos (doom+chromatic orb), wyvern (+4) shapechange, sword spider (+5 AND poison) transform, webs, chain lightnings (goodbye druid lightning which requires outside only). Also there are NO cons. You do not create the character in 2 anew, no penalties (assuming you played with a carried over druid in 1), if you choose the kit with a BG1 Druid the Ankheg Armor is carried over AND worn (equipable), and being able to wear the black dragon scales totally destroys the "only leather armors" con.
5) Wild mages. These overpowered beasts defy all game rules and logic. A chain contigency with 3 greater chaos shields obliterate their risky nature and unpredictable casts, entirely! A reckless dewomer, makes you use ANY scribed spell WITHOUT prior memorization. A carefully timed timespot, coupled with wish to get a quick rest, or a limited wish to try and replenish your spells, ends to an endless cycle of endlessly casting and recycling spells, without resting, memorizing or any chance to screw everything and everyone up with a crappy wild surge. And all those wrapped up with the ability to cast more spells than your average mage. Throw a robes of vecna and an amulet of power on, and this thing is so gamebreaking, it is not even funny!
Now your turn.