My first game I played with a Volus engineer, I swear someone was missile glitching. All I could hear and see were those HUGE explosions, and I had no idea what was making them. Eventually I saw the little guy laying the mine. Thats such a big boom for a little guy
Also, Multi-Frag is the ultimate "osh*t" button. Please try him with a Revenant if you haven't already, you are Rambo.
My favourite of all the newer stuff is still the Paladin though, I just love smacking guys with the shield and standing right in front of Primes and whole groups just soaking the damage. It is the best when your team actually takes the hint, and just stands behind you guns blazing (especially good when its a Destroyer with a big gun).