I'm looking for class advice assistance.

Hi there...

I'm going into Legion KNOWING that I'm going to have to "main" 2 classes: 1 for PvE content and 1 for PvP. My PvE toon is going to be a Blood DK. I've tried (and dislike) UH and Frost (especially the piss-poor kit that Frost has been given...)

What I need help with is deciding on a 2nd "main" for PvP. I plan on doing arenas aggressively with 2 buddies (resto druid and enhance shaman). Could you kind, talented folks please recommend what a great compliment to those 2 classes would be? Honestly, if it ends up being Frost DK or UH DK, I wouldn't mind... I was the only one of the three of us to get into the beta, so we weren't able to play around much with the composition, and they're both dead set in what they want to play.

Thanks much for the advice in advance.

EDIT: To clarify, I have 1 of every class at 100 and am willing to play anything, so long as it compliments the 3's arrangement.