Democratic detonation's definitely the top dawg. In addition to it though, it really depends on what you want to do, every warbond kinda has its own theme. All I can say is stay away from the crossovers, because they really don't add much to the game. I personally love viper commando for the armor and booster, but I like to be in the thick of it with enemies.
For solo play, urban legends has the antitank enplacement, which is really fun on bot missions. Control group has the warp pack which lets you access bunkers on your own. Chemical agents has the gas grenades, which is good for crowd control. Struggling to think of more solo play warbonds, maybe the stealth one that came out recently?