Well, I have only played MK1 so far (My shop is going to prolly switch right after the current escelation) so my experience lies in what worked in MK1. Let me just start with this: My first purchase was Beast-09 and then Vladimir, Dark Prince and I find them both ending up in every single list I make. Beast just has tons of strengths. Infantry? Beast has Thresher and Imprint: Murderous. Jacks? P+S 19 axe and an open fist to throw lights. Snipers? Beast takes an extra movement if they try. The only times he ever fails me is against Cygnar if he gets scrambled. Other than that, he's a force of nature.
Vlad is getting toned down in MK2, but will probably be still effective. Just not as broken. I haven't had much experience with the Butcher but from what I can see, he's a strait up badass that likes lots of 'Jacks. Your opponents let him get a charge off on their warcaster exactly once, and then he should have a 10" deadzone that nobody wants to place dudes. (5" movement + 3" for charge +2" for Reach is 10")
As for Jacks, Kodiak is a fine addition to any force. Two is brutal in MK2 as the first one combo throws the target and the second one charges and Hammerfists for extra
d6. Add onto that Vent Steam for infantry and All Terrain and the Kodiak is an excellent 'jack.
Devastator also works well against infantry. Its attack dosn't need to hit and delivers instant POW 18 goodness that can then be boosted. Its
ARM 25 renders it nigh immune to your foes attempts to kill it before it can go off. This was my second 'jack purchase and I find I win better playing it.
The Juggurnaut I have mixed feelings about. For me he's a cheap 'jack with a strong attack that usually ends up recieving more charges than he makes. For almost the same points I'd rather run a squad of Widowmakers, Kell Bailoch and Eiress. Its only 4 points more!
Destroyer is a good 'jack, but since it fills the same number of points in MK1 as Beast, I havn't really played it much. There is a lot to be said about a Kodiak throwing someone to the ground and then Bombarding the holly hell out of it from a range with one of these and a couple Winterguard Morter Crews.
I have never touched the Marauder, so I have no idea about it.
If you want to run tones of 'jacks might I suggest Karchev the Terrible? He got some major tech in MK2 and all his jacks got better. With him you want the Repair Crews and the War Dog, no question.
Hope this helps, sorry for the wall of text