You made a good point ygg. If a kill-team isn't well equipped to deal with armour, they probably should avoid it!
Damage output, like so many things military related, depends upon the vehicle and the role it is meant to play.
A kill-team isn't going to have a tank company out hunting for them. That is, unless they stole the trigger switch for the ultimate weapon the traitorous renegades need to destroy 90% of the Imperial Navy in orbit around them thats about to drop 18 different degrees of Hell upon them.
Even if your kill-team blows up a depot or something, your typical military response is to send out an appropriate scout, recon, or reactionary unit. Probably a Sentinel squad and/or a chimera or two with scout unit, or a valkyrie and gunship escort to do a hunt and retaliate mission. Only if the kill-team beats those and continues to be a MAJOR pain in the butt is your average military leader going to escalate. Seriously, a couple of vehicles and back-water depots attacked doesn't really warrant a full scale war, not when retaking and remanning them is a trivial matter for an army.
The kill-team going and/or attacking places or military units with tanks, thats another thing entirely. If you go poking a bee hive, don't be suprised when the bees poke back.
As for the vehicles themselves, I think the most damage output against infantry would be anything loaded up with heavy bolters or auto-cannons. These are weapons designed to mow through infantry (autocannons being a multi-purpose weapon), they have damage which can touch marines and also spew out enough shots to over come dodges and shred defenses in very little time. Flamers are also good anti-infantry but they are designed for lightly armored units, not so effective against space marines. A Leman Russ exterminator or a pack or auto cannon Sentinels would likely put the hurt on space marines. Multi-lasers (chimeras and sentinels) would also be effective but their lower damage output makes them less effective against marines.
Tanks with bigger guns run into several problems when facing infantry (even power armoured ones). First, shooting high-powered rounds at infantry kind of goes against doctrine. I mean thats what the heavy bolters are for right? Second, if you miss, well you miss. One shot every few seconds has fewer chances to hit (and thus do damage) than a constant stream of lead (especially a BS 30 gunner firing from a movinge vehicle: -10, these rounds were meant for other tanks, ones that give a +30 to hit and such). Third, an average dodge by space marines will avoid that lascannon or battle cannon shot. Thats not to say they can't hurt a space marine. Indeed if one of these shots, designed to put really big holes into a tank, hits an average marine, that marine is going to feel it in the morning. A Battle cannon is pretty devastating (it even has that rule!) and a single hit could cause a squad a lot of cohesion damage. A vanquisher round would put a marine into crit at the least, if not outright kill him, same for a decent lascannon hit. Lets not even go into a D-weapon hit… Turbolasers - fugetaboutit!
Artillery barages are mean, scary, and deadly. Even if you aren't a huge threat a general might just decide to lob a few shells your way. When I say "few" I mean three to five rounds (if not more) from anywhere from couple of guns to an entire company or more. (Its much cheaper to fling out 30 high explosive arty shells than use up the gass, time, and ammo of a tank company.) What makes arty even scarrier, you'll never know its coming. Even worse, if its indirect you can't possibly be aware of the attacks. The best you can hope for is to skedaddle out of the target area or find some damn good cover. On the plus side, the arty units would be shooting an area, not your kill-team specifically. They may hit close, they may be off by miles. They may have a nice concentrated shelling, or they coud be uncoordinated and dispersed.
Then there's aircraft. Honestly I'd love to see an Imperial Navy supplement for OW. Just to see if there are any new rules for bombing and strafing runs. Aircraft can be a MASSIVE threat. Striking at your kill team from altitude where the kill-team can't effectively counter-attack. I think aircraft are better as a non-structured threat and encounter. With few exceptions theres really not much you can do about a loitering aircraft, and its only going to be in range when it rolls into a pass at your marines.
When you are building your hostile or potentially hostile forces it helps to flesh them out. Decide how many of your armour units are equipped in what way. How does your military handle scout squads? What do they use for retaliator or reaction squads if anything? Which vehicles and what armament do they prefer? What kind of stuff does the Kill-team have to do to really piss off a general into sending in the big guns? Is the enemy general going to modify his tactics once he learns he's facing off against space marines? Answer thee questions ahead of time and you'll have a much easier go at determining armour forces.
Hope that helps.