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.
Solo I tend to pick scout just for laziness of getting around quickly. Normally only run simple mission types this way.
In group play I mostly do driller. Even if I'm useless the whole mission I can make up for it with B lining to extraction point. Overall I just like the feel of the class and that I am always making an easy to use hole to where we're going and use later. (Don't worry about the one that leads through a ceiling to my and your death. I'm sure it won't be a problem later in the mission.)
That said I do like all the classes and mix in a healthy share of gunner and engi keeping overall ranks semi near each other.