What is the best all around character class and how should I create him at the begining?

The most powerful class is probably the kensai, dual classed to mage at level 9. He should be a master of dual wield, and probably should be good at katanas, possibily long swords or axes as well.

The most useful class would be some combination of thief (i.e., besereker dualled to thief at level 9; pure assasin or swashbuckler; or multiclass thief-fighter), unless you don't mind keeping Jan Jansen the entire game.

The reasoning behind these choices is there are only two good thieves in the game and only three pure mages in the game and you really cannot do without level 7+ mage spells or good thieving skills in this game.

Also very good would be a ranger dual classed to cleric at around level 9. Make the ranger specialize in hammers, flails, and slings (if you have an extra point put it in dual wield). Some great flails early on in the game and the best weapon in the game is a hammer. With all these characters make sure they have 18/91+ strength (unless a pure thief kit).

Who are the good thieves and mages in the game?

Do you think its better to start with a multiclass fighter/thief or dual a swashbuckler? If swashbuckler, what should I start with and when should I dual class him?

The good thieves are Jan Jansen and Yoshimo. Imoen is fairly good but she will have to drink master thievery potions in some situations. Care must be taken so as not to lose Yoshimo in chapter 4. If you want a spoiler someone can tell you what you have to do.

Edwin, Imoen, and Nalia are the good mages. Edwin is the best but is evil. Obviously you can try to kill two birds with one stone with Imoen but you are going to have to wait a long time to get her.

Fighter-thief is probably better than a swashbuckler because he can wear plate and still detect traps (although there are plenty of good leather armors in the game). If you went the way of the swashbuckler, I wouldn't dual him because it takes too long to get the thief skills back (fighters need lots of exp) and he will still have a lower strength than a fighter.

Actually, maybe a swashbuckling mage would be the best combo. There are plenty of strong fighter characters in the game, a swashbuckler will definitely be able to fill all roles (front line, thieving and mage).