Recently, at least, my main has been Scout. I run M1000 with Electrocuting Focus Shots - the way I've built it provides a nice breakpoint for acid spitters. After playing Scout for a while I find it a little hard to go back to a slower class - more than once I've tried to grapple away and realize "wait you idiot you're playing Engineer".
After Scout, I like playing Gunner the most, followed by Engineer. I don't really like playing Driller much - despite having many promotions on my other classes, I haven't promoted Driller yet! I don't hate the class, or anything, I just didn't ever really click with the playstyle. I have a healthy respect of Drillers on my team for being able to handle crowd control well, so cheers to you guys.
Im absolutely terrible at traversing terrain and being able to just look at where the drop pod has landed and tunnel my way there is what made me love the class.
The flamethrower is great while solo, and the Cryo gun fixes the issue of friendly fire while playing with friends who can all gun down the frozen dreadnaughts nice and quick