I don't play a huge amount and tend to solo a fair bit, but from what I've seen and in my opinion:
1/what characters are the easiest to play
None are enormously difficult but play style changes dramatically between them, Runies and Hunters seem level fastest.
2/whats the best healers? (thinking groups)
Minstrel
3/whats the best solo class at end game
Personally I don't think there's one class that's madly better than any other, Runies are like glass cannons, but then again Hunters kill reasonably fast too and are a tad tougher. Then you have the Wardens and Guardians which kill slowly but can survive far better.
4/whats the best stealth, class an is it worth building them up,
There only is one - Burglar, hard to level compared to the rest but they are great fun. Hunters have stealth, but can't move whilst hidden(camoflage only).
5/whats the least played character (and why?)
I don't play often enough to notice a least played class to be honest. I know I see slightly less Burglars and Loremasters now than I use to.