Engineer? Best class? No sir, not at all. That's the Drell, Vanguard or Adept.
I play Sentinel, which is probably the 2nd best supporting class (after the Infiltrator). Access to Warp for peeling away enemy defences (which is incredibly useful for those Phantoms), Throw for totally negating Guardian shields (and generally being a spam attack), and a permanent 25-45% damage reduction from the tech armour.
The Drell Vanguard with a shotgun + bayonet is obscenely powerful. Charge into enemy, straight into melee, instant health bar gone for the enemy (apart from the Atlas, of course). Charge recharges straight away, so you can either escape if it's an Atlas or Guardian, or spam it and the extended melee to more or less OHK anything.
Then, there's the Cluster Grenades. Once they are maxed, they can halve the number of enemies in a few throws. They aren't as damaging as the Frags of course, but they make up for it with hilarious ragdolling and larger coverage.
The Drell Adept is just mental. Reave + Cluster Grenades + Modded Carnifex/Phalanx pistol, and you're nigh-invincible.
Anyway, I too am impressed - Or at least, not disappointed. I expected a tacky, rammed-in multiplayer like Bioshock 2's or so on. Instead, I got an entertaining and challenging (on higher difficulty) horde mode with RPG elements (and not just useless levelling, either).
Good job, BioWare. Now as long as you haven't made the singleplayer campaign rife with clichés and bad choices, you've done us proud. The combat and RPG-ness is significantly improved over ME2, and you've removed some of the awful design choices made between ME and ME2.