SIth Juggernaught. Awesome story, Really fun class, Getting hella nerfed
in vengance tree come 2.0. Best animations in game. Cool armor. AND FORCE PUSH! Honestly it's one of the most fun classes in game, as well as diverse in playing tank hybrids/dps and will be even better in expac. Since hes imperial you'll have contact with your toons. I think it would be win win. My second choice is sniper or PT but you already have those
Don't you mean buffed? My Veng Jugg is doing pretty nice compared to my Watchman Sentinel on the PTS, better than the difference on the Live Servers.
But yeah, a Knight or Warrior would be good ideas for leveling next. I would rate their Stories as equal and as very good. Both are different kind of Stories than what you have done already, have a much different playstyle and resource system. I would recommend the Knight because it allows you to do the Republic Side Quests and see that perspective, instead of the Imperial ones for the fifth time.