When he ask for the BEST character class, I understand that he is talking about POWER between the class; of course the best character class about PLAY is the one that you identify with.
Supose you mean POWER between the class(that is also based on personal preference, but a little less):
The classes have capabilities of their own( ohhhhhh

!)

. kidding. The thing is that these capabilities couteract thenselfs (like a pentagram circle, or elements circle [ the 1 beats the 2, that beats the 3, that beats the 4, that beats the 1,...]). So suposing this is right the class with more capabilities will have more counteracts and less vulnerability. So from the start I should say it is probably a Multi/dual class.
This multi/dual should be compesed with the two single class with more capabilities, in my point of view it should be a Mage/Cleric ( because of the spells variety and possiblity of counter), BUT even this combination (like all others) will have similar fragilities from their root single class( a fragility of the mage that is also from the cleric, meaning that is a fragility of Mage/cleric), for example: a thief with cloak of non-detection will fool the the Mage/Cleric.
I mean that will always be a way to counter some combination or strat. I think is possible to be a better class against an especificaly other. But not a better class against the general. In the case of a Mage/Cleric probably something with stealth and cloak of non-detection.
My personal opinion with most conteracts and less fragility still the Mage/Cleric.
But this confusion must be also added with some more variables: Like level, equip, spell chosen(sorcerer), spell memorized(mage/ cleric/ druid), abilities chosen,...
I'm realy confused now.
I am remember when I was 12 years old, I ask to a Karate master, what is the most eficient matial art(stupid question). He say in a serenity way( hate when they do it ) : "There is no better martial art, there is better apprentice." (There is no better class, there is better player [or better cheese.

] )
Sorry about my english, sorry to everybody who try to understand it and fail; is realy dificult to say such long things in other idiom.