Well... if you want to follow the 'crowd', the easiest would the infamous Kensai->Mage. Berserker->Clerics are notable as well. They appeal to those who like to melee while boosted by spells.
For those who prefer magic... Conjurers, Necromancers, Generalistic Mages, and Sorcerers are the way to go.
For those who want style, pride and honour, Cavalier, Undead Hunter and the Inquisitor.
For those who just want to bash things and could not be bother to fiddle around with spellcasting, we have the Monk, Barbarian and the Berserker.
For those who prefer stealthy combat, we have the Archer and all the Thief kits. The Kensai->Thief is particularly awesome here if you have ToB.
Then there are those who like to multiclass and try to get the best of 2-3 worlds. Fighter/Mages are like Kensai->Mages, but with less melee and magic power. They make up for this with more HLAs from ToB.
Cleric/Mages are the powerhouse of spellcasters, although they will never excel in one class, they compensate by having a mind-boggling array of lower level spells.
Ranger/Clerics are for those who prefer beneficial spells, as you are essentially marrying a tank with a priest. This class gets the benefit of having Druid spells as well.
In the end... the class that you will find the easiest to play with, is the one that fits your style of playing. Frankly, I suck at soloing kits like Barbarians, Berserkers, etc... but other people have had great success with them. I prefer raining ammageddeon(sp?) on my enemies with my Sorcerer.
