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.
I mainly play engineer both for the versatility and for the ability to place turrets and platforms. The platforms in particular open up a lot of options for traversal. The engineer also has access to some of the coolest weapons in the game in my opinion.