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.
Ive played all the classes for about 100hr each. Last class i chose was driller mainly because i dont like the feeling of flamethrowers in games and my friend was already playing as them. Anyway i level up with driller until i unlock the (imo) best primary in the game: Corrosive sludge pump. GODDAM DOES IT FEEL GOOD TO USE. I can charge a shot and the bugs fizzle away like soda. I now have about twice as many hours on driller as other classes, currently on silver 1. Driller is the most fun i had in his game