Let's see if we can get some discussion going.
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.
Engineer. Engineer has the most creative gameplay I find. There are lots of ways of using his kit to create chokepoints, save ammunition by electricuting enemies without shooting them directly or with turrets, bridges to nowhere to funnel enemies that you can jump off and make them walk all the way back, and more.
Scout is my go to when I want to rely on speed and skill over excessive/quick planning.