General Supremo gets a lot of flak since he can't jump while firing due to his All-Star style weapon but I've done quite well with him in PvE and multiplayer. One of my favorite Foot Soldiers really.
Scuba Solider's ability hardly ever seems to actually help much, although his damage is solid. I would love if they kept BfN's spotting system over GW and GW2's and made this ability more generally useful for the team.
Plumber is my favorite engineer. Yeah, he's not amazing or all that different from the baseline Engineer, but he's amusing and effective.
Landscaper is decent too. I don't see him all that often, but he's better than he may appear at first glance, and indeed he sounds like someone is trying to start a weed whacker.
