Best solo... hmmm... well my opinion is witch doctor. Pets are tanks now. Gearing isn't expensive. I too started out with wizard and like the class. But was always frustrated at not being able to hold up in a fight. With the pets out, my WD can hold up against mobs that would crush any of my other chars including my barb. Course that has more to do with my gear at the moment, but still :).
Barb is also very good though you are stuck in one or two builds if you want to really run high MP. Course that can be said of all the chars at the moment. Hopefully the new item stuff is going to open up more potential, but I'm not optimistic about this first pass. Maybe after another iteration or so.
You can solo all you want and up paragon with almost any class / build you can think of. Playing MP5+ is another issue, though...
He want's "the best" though.
I'd say the best is what you feel more comfortable, and have fun using... But, nah, let's stick to the staple answer: I second the heli-barb. --- Currently playing:White Knight Chronicles 2 [PS3], Diablo 3 [PC]