Engineer.
Bear in mind, I formed most of my opinions on this in ME2, which I still think is the overall best game of the series.
I used to play sentinel a lot because I figured it was good to have some to deal with anything yourself rather than having to rely on your squadmates and their derpy AI to take down shields or barriers. Plus, when I tried to play an adept I found myself missing the lifesaing properties of tech armour on higher difficulty levels.
Then I played engineer having been turned off tech powers by their mediocreness in ME1, and holy shit.. it was awesome. I went demolisher, I maxed out incinerate, cryogenics and overload and got myself an exploding robot friend, and the entire game was just non-stop power spamming fun. No long cooldowns, lots of dealing massive damage and exploding frozen people with tech combos, and best of all low cost research meant less time spent dragging the scanner around random planets.
It's very difficult to play any other class now.
I find myself wanting to replay my engineer playthrough just because the combat was the most fun. Then again, I'm probably more of a tactical combat person so my definition of fun may be in the minority.