I need video of Steve D annihilating Goss in a game of Warmachine. How awesome would that be?
Ian, now I'll NEVER use anyone but eGaspy.
On a serious note, you need to put down what version (epic or prime) so we can help you more, as they play fairly differently.
Overall, here's some rundown of what I know:
eHaley- she has a feat that is unbearable, other than that... I feel she runs more of the support game.
pHaley- still more support but it's way more straight forward to play.
pStryker- simple guy. Simple stuff.
eStryker- one of your better assassination casters, on good circumstances can one shot casters.
Brisbane- if it's siege, go with what rich said.
pnemo- warjacks, probablly thunderhead and two more.
eNemo- MOAR JACKS!!!
Units:
stormBLADES- assault is awesome, not commonly used for Jack marshalling, but they are a common unit and a "staple purchase"
stormGUARD- similar but have reach and electroleap. All in personal preference.
Gun mages- solid tricks bag and best Jack marshal unit in most cases.
Long gunners- with the ua, they pick off stuff, but really lack once the opponent is on top of you.
Trenchers- a little all over the place, but solid. Lots of abilities to frustrate the opponent, te ua makes them really good if you want to run a more ranged oriented army. Watch Ian with his winterguard, similar deal.
Sword knights- good with lots of jacks
precurser knights- good melée unit. Better with Blaize.
Nyss hunters + Murdoch - OMFG SOOO GOOD. pew pew dig dig, what??? Pow right in the kissa.
Jacks(the good ones)
gallant (with Blaize, still good without)
ironclad (cheap heavy)
stormclad (argued as your best if you run a storm unit)
thunderhead (needs no explaning)
ol' rowdy- really nice Jack to have playing net for your caster.
Minute man- best light EVER
lancer- really solid light
thorn- ( character lancer)
hunter- (nice for solo hunting and against jacks)
avoid fireflys at any cost and when out the avenger they are awful.