Becouze Solo RvR means your only Targets are Stealther, i vote for Cleric...

In my opinion, best solo caster is an elven nightshade on uthgard!

If you pick a staff in the 2h slot, youll almost look like a caster!
Youve even got a DD spell, NICE!
You still can run around solo, at least stealthed!
You are a hib so you wont have to face a stealthed fg full of rangers at any milegate!
And as CC, youve got your sidestun and a snare(at least from the envenom line) so youre as good as any other usual caster having 2 CCs.
And do not forget the most important part, you can easily find groups in emain!