Mix between the Engineer or Soldier.
I enjoy synergy with combo explosions and weapon usage. Adepts, Sentinels, Infiltrators, and Vanguards do this very well, at varying ranges.
The Engineer however, seems to rely on Drones, which while useful, seem awfully passive. I tried the Drone Army build, using 3 drones, but didn't feel the impact.
The Soldier seems to have only two options - keep shooting, or spam concussive shot. The reliance on ammo powers further makes this class passive.
The Adept is an extremely active class, as every power has it's use. Warp a single target, then Throw or Shockwave. Singularity a cluster of mooks, then Throw or Shockwave. While it also seems to be two options, the explosions make it feel active.
Sentinel is the toolkit character, which I usually play with all 4 powers - Throw, Warp, Overload, and Cryo Blast. Actively strip defenses, then trigger the explosion, and profit. Tech Armor is just icing on the cake, letting you play more aggressively. It's also the only class that can have two protection powers running simultaneously, for some awesome damage resistance, letting you run it as a partial or complete weapon class.
Vanguard, while has it's use for spamming, is about close range overwhelming force. Charge into an explosion, flank or rear end your enemy, and let the disarray from your enemies be your advantage. Charging, depending on the difficulty, is either boringly fun or risky and rewarding. I tend to play it with light weapons so I always have options.
Infiltrator is my favourite, but only because it fits my playstyle. It's precise force at any range or position. While I used to use it exclusively with Sniper rifles, I enjoy changing it up with SMG's, Pistols, and Assault Rifles, in a nimble tactical way.
Between Engineer and Soldier, I'd have to say Engineer is the most difficult. Drone's are it's forte, and while they are good, they aren't 100% reliable.