I have done Fire/ Marine/ Fire and Arsenal/ Marine/ Mace, and while both characters are fun, I like the Arsenal/ Marine better.
Fire/ Marine/ Fire: With the PBAoE powers and Whitecap, this character likes to throw Flashfire into a Tide Pool'd spawn and then Whitecap in, Cinder, and mop up. Fire Imps Love Marine.
Arsenal/ Marine/ Mace: This character sneaks up on a spawn with Cloaking Device, Tide Pools, and then opens up with Smoke Canister+Sleep Grenade+Tear Gas to completely bamboozle the spawn, and then his Tri-Cannon and Toxic Tarantula (both of which love Marine) get to work.