I've had nothing but success with using a blood angels 10man
tac squad with heavy flamer, flamer, combi-flamer in a rhino. They can move across the board at a fast pace (18" every turn) or can fire 2 flamers out of the rhino every turn. (they can also wall of death while still inside the rhino.) These squads usually end up wracking up more kills than my
DC or
SG.
You can also combat squad them and drop 5 marines with bolters off at an objective then head off to flame the enemies back fields. Most of the time they leave squishy things hiding in cover to hold back objectives.
The rhino is also amazing at Tank Shocking models into piles of bodies to roast
The marines aren't that bad at combat either, they can put out the hurt on the charge and then hold their own in combat for a few rounds, being Obj Sec. allows them to charge stuff camping on objectives and sit in
CC for 3-4 turns holding the objective over the enemy.