The Baron. The Baron is what makes Hellions excellent. +1 to your cover saves (probably the most important part) and re-rolling dangerous terrain is great as it is, then he adds S6 attacks, a phantasm grenade launcher, and gives you +1 to the roll to go first. Oh, and did I mention he makes them scoring?
15 is usually the best squad size. Big enough to take some shots and still make it there, but small enough to hide without too much difficulty. The Baron means it shouldn't be too hard to get a 3+ cover save so that you can make it there. They can put out masses of accurate firepower at range and due to said save can survive a bit of retaliation, so don't be fooled into thinking that just because the hellglaive is more eye-catching that they can't win in a firefight. As for combat, S4 with that many attacks will kill most things, and then you factor in combat drugs and pain tokens.
They really can do kill practically anything except tanks, and even then the Baron can do a number on light tanks, as can the hellions if you roll +1 strength or get 2 pain tokens (or even both, that's S6 Hellions!)
I'd take a Helliarch with agoniser but I expect they can probably do okay without it. Obviously the best case scenario is killing the enemy in two turns, at which point you can go for another unit, but remember that you have Hit & run, so if you're about to be counter-charged you can get out of there.
"Oh how awful, did he at least die painlessly? To shreds you say? Well, how's his Dracon holding up? To shreds you say? Very well then... Sad, sad, terrible gruesome news about my colleague Archon Mhu'bhutu." - 'The Feather', Dracon of the Bladed Lotus