It's been a long known fact that the Sorcerer is end of any discussion about the "best" class in the BG games is - but people only come to that conclusion once they know the spells of BG II like the back of their hands.

As sorcerer with the right spell selection can solo anything in the game - in fact some guy did a complete speed run of BG II with a sorcerer in about 30 min :D

30 minutes? How could you possibly do that on a 'complete' speed run?

Actually the current record is 23:09 minutes :D - http://www.youtube.com/watch?v=o2sMvrFSvDM

For me, the best class is the class that's most fun to play, not necessarily the class that can use the best weapons or abilities.

I like ranger and fighters type the most but that's just me

I actually like paladins. They're good front-line tanks and get nice saving throw bonuses. Carsomyr (probably didn't spell that sword right) is icing on the cake.

Bard is the best class :) Just look at Baldur's Gate 1 bard solo youtube video. Bards have so many options, are versatile and gain levels extremely fast. And this is viable class even in BG2, cause of Blade. Just check it out! :)

My question is: when ( at level) should one dual class my main character, a human fighter?

Level 7 for 1.5 attacks. Level 13 for 2 attacks. Anything after 13 you would only be going for THAC0 and/or extra proficiency slots. Not worth it, since you need to get your character's main class back ASAP.

Most powerful through both games together is the multiclass. If you insist on playing a human dualing at level 6,7 or 9 is a good choice.

yep the multiclass is the best based on how experience is given in BG rules. You can be a wizard, or a fighter/wizard with just as good casting ability and able to survive and use weapons easy. A cleric, or a fighter/cleric dual welding warhammers and casting cleric spells like any other cleric. A figher/thief who can thief like the best of them, but can dual class +5 daggers chopping through enemy mage defenses like butter as good as any warrior. Etc.

Worth noting that if you're using the original engine, if you go pure Druid in BG1 you can pick up the Avenger kit without stat lowering when porting to BG2. This basically means free mage spells and shapeshifting forms.

It turns out that AD&D based games actually do have best classes, largely due to the fact that any given game's environment tends to favor certain classes. Here are some of them (considering only single classes):

Baldur's Gate: Fighter with a focus on bows. (Slow walking speed + availability of powerful arrows + spells being limited at low levels and area spells being hard to use.)

Baldur's Gate 2: Wizard or (if you're already familiar with the game's spells) Sorcerer. There are tons of powerful arcane spells, including some that you don't get in other AD&D CRPGs, like (Chain) Contingency, Project Image (which I believe is more powerful than its AD&D counterpart, and is almost as strong as the Phantom Warrior spell from Romancing SaGa), Time Stop (intended to appear in a couple SSI games, but not always available/functional), and even Wish. Also, if you are willing to use scrolls (which are plentiful, though typically finite) as consumables, you can also get around the game's level gating of higher level spells (or you could use wild magic for this).

Icewind Dale: Cleric. Undead are common in the game, and Clerics, unlike Wizards, are not affected by the scarcity of scrolls and lack of level-up spell picks.

For some of the older SSI games:

Pools of Darkness: I believe it's magic-users here, especially since, at high levels, they get the ability to pierce magic resistance. The spell selection is rather lacking at some spell levels (there's no spell above 7th level that's as good as Delayed Blast Fireball, for example).

Dungeon Hack: Cleric. Wizards have the same issues as in Icewind Dale (plus the fact that the spell you want isn't even guaranteed to be in the dungeon), while Clerics get spells to negate many of the game's dangers, including hunger and poison.

Dark Sun: Shattered Lands: Probably Psionic. Psionic powers are not level gated the way spells are. You can, for example, get Mass Dominate after the first level up and break the game that way. (I highly suspect that a speedrunner's party would include at least one psionic for this reason alone.)