Cygnar - Stormwall
Easily the best colossal for two reasons. One has got to be covering fire on the secondary weapons. Covering fire is awesome, two templates with POW 12 really allow you to control the board at least with respect to infantry and can cause issues with beasts and jacks as well, a damaged beast/jack has to be wary of losing an aspect when moving through covering fire, that it's on the secondary weapons and you can still take two big gun shots a turn makes the ability really sing. The other has to be the pod, there are so many ways this is helpful from making sure your stormsmith stormcallers always get off triangulations to giving you a target for spells and abilities like chain lightning, earthquake, Mage Storm and electroleaping weapons. A little explanation for mage storm, when you don't have enemies you can affect with a pod what you can do is leave a 4" gap between your two covering fire templates and drop a pod in the middle, then have Ryan (black 13th gun mages) Mage Storm attack the pod. It means you can have 10 inches across of auto POW 12 templates which is really good.
Best Warcaster - Haley2 (though it can find a home in really any caster's list, it's just that good).
Impact - The impact of this jack in faction is massive, it doesn't bring anything new to the faction but it does what the other factions jacks do but better. It shoots and can melee, something Cygnar sorely needed, it's got great defensive and aggressive abiliities and it makes Cygnar's signature spell (arcane shield) really sing.
Galleon - Mercs
The second best colossal in my opinion though for very different reasons than the Stormwall. The Galleon shoots and fights well. It can put out a lot of AOEs but that number is random and it doesn't really have a great deal of special abilities. Drag is nice but there are a lot of ways to avoid it. You can use terrain, armlock your own models, sit behind cheap screening infantry etc.
Best Warcaster - Broadsides Bart. Again though it's good with pretty much everyone.
Impact - This is what makes the Galleon so good. Merc jacks have two problems; they suck at shooting and they are low P+S, the Galleon is great at shooting and has high P+S on both melee weapons. The Mariner requires a loader which is painful to set up and the mule has very short range unless it aims, the new releases in wrath helped a bit but it's only really with the Galleon that Mercs finally picked up a really good ranged jack. Previously all Merc jacks (except the Rockram) topped out at P+S 17 where as now we are up to 21 which means Mercs can now crack high ARM. Add to that it brings much needed survivability, merc jacks are generally the least survivable, the base stat lines (general stat lines) for the other factions are Khador jacks 10/20, Menoth 11/19, Cygnar 12/18, and Cryx 13/17 (12/18 for the crab jacks). You can see the pattern, Merc jacks are 10/18 or 12/17 (pirate) and they don't have a lot of damage boxes either (same as Cygnar jacks, the second lowest, sorry Cryx). This jack fills a huge hole in the Merc line up, gives the mercs an option for dealing with high ARM, a way to deal with stealth (AOEs), and a truely survivable piece to take a control zone. Most casters will love it and it has really changed the faction.
Judicator - Menoth
First one of the Colossals that I think isn't particularly good. Its stats and abilities are all right except for the fact that it has inaccurate on its rocket pods (bringing it to a mighty RAT 1 except on its sprays) but it puts out four AOEs a turn without focus and the blast damage is POW 9 with the choir. It can also aim and with the choir be back up to RAT 5. Add the choir to its sprays and you will devastate infantry and it's quite good in melee. It picks up a focus for free too which is good but not spectacular.
Best Caster - Feora2, it's AOEs will set everyone on fire and Feora uses that for her feat, she can give it four focus and she speeds up her jacks which is pretty important for colossals (which generally want to shoot rather than run).
Impact - Very little. The problem is that you can't enliven a colossal. That said you can always throw an ARM buff on it and enliven two other jacks but it means that your most expensive piece is without your best defensive trick. The other problem is that the existing Menoth jacks are just too good, unless you have a specific synergy or a specific role for the Judicator generally you are better off taking the menoth heavies. That said there are some great synergies in addition to the one with Feora mentioned above. Expect to see this jack just not in most lists like the Stormwall and Galleon.
Conquest - Khador
This one is OK not great but certainly not bad. Crit devastation is fun, the gun has a good POW and the secondary batteries are occasionally pretty good but creeping barrage is not covering fire and the Jack lacks other abilities.The jack is saved by its massive amount of damage boxes, P+S 22, the cheapness and effectiveness of Khador mechanics and the fact that regular khador jacks are not that great.
Best Caster - ? I'm going to go Sorscha2 because she does the most for the jack but the jack doesn't really do a lot for her. Maybe either Vlad or Irusk1.
Impact - Khador has always been an infantry faction. The jacks are too slow, too bad at shooting (except for maybe the Behemoth) and lack survivability (I know it sounds weird to say that as they have the most survivable base stat line but their base stat line is killable and they have no tricks to keep their jacks alive). The Conquest fixes these problems. Not quite as hard as it does for Mercs but still it's a solid improvement, it does what the Khador jacks already do just much better. As such expect to see it in quite a few lists, although as an infantry faction there will still be plenty of players taking a cheaper jacks and spending the points on infantry.
Krakken - Cryx
On paper the jack look unwieldy, requiring corpse tokens to function at maximum efficiency but it's actually really good without the corpse tokens too. Cryx can buff the crap out of this thing or debuff whatever it wants to destroy. It has massive threat range, its gun shoots forever has a good size AOE and can shoot after it kills something in melee.
Best Caster - another ? Skarre1? Goreshade2? Mortenebra? Witch Coven? I don't think I can call this one but I think it's one of this lot.
Impact - Cryx lacks ranged attacks and Cryx jacks outside of the death jack and the great cheap lights leave a bit to be desired. The Krakken gives cryx a survivable jacks with great ranged capabilities that is also fantastic in melee. That said Cryx is still an infantry faction so a lot of casters probably prefer the points to be spent on infantry but the Krakken can work its way into a lot of lists.
Hyperion - Retribution
Back to a jack I think is straight up good. POW 18 on a ranged attack is fantastic, add in a full size AOE and critical consume and the gun is silly good. It has two smaller guns with the colossals standard d3 attacks and is quite good in melee too. It has the most damage boxes of anything in the game and Ret casters have a lot of buffs they can put on it.
Best Caster - I'm going to have to say Vyros1 he can increase its speed, increase its ARM and let it shoot straight through models and terrain to whatever target it wants.
Impact - Ret doesn't have a lot of good jacks. The Pheonix is good and the Banshee is good and some other jacks have specific synergies that make up for their "not generally that goodness" but in general Ret jacks are a little lacking, paying a high points cost for their versatility while not being able to do anything particularly well. Hyperion though is just plain good. Ret also has cheap mechanics that can also give the jack +2 on melee damage and they should always be in the list with Hyperion.
Mountain King - Trolls
Joking aside I might do one for the gargantuans when I know what's going on with the Everblight and Skorne ones.