On nod - Banshees, Infernos, Bikes (squads make bikes shockingly good against all slow firing tech units), scorpion tanks, Basilisks (rarely these days though, nod tech isn't often run), Kane's obelisk with some blockers in front of it.
On GDI - Jumpjets (mammoths can run them over but multiple squads can still take one down without too much trouble), predators, mlrs with blockers, titans, kodiaks (testing these still this patch, they haven't been great in previous patches but I think the recent banshee and talon nerfs leaves a bit of room for them), sometimes I'll throw a few orcas at them combined with other things - orcas aren't really strong enough to take mammoth's on their own but they do great added damage, Titans also do good work especially when you consider you can get two of them for the cost of a mammoth pretty much, and every so often - talons, yes, really, mammoth aa is bad enough that I have killed mammoths with talons before :P
Really though the biggest trick against mammoths is to apply a ton of pressure throughout the game, they are very hard to get out, almost nobody played them in tib league at 200, they're pretty awful at 250, and really we're not actually talking 250, their real cost is 370 because honestly in a high pressure game you won't likely have the tib to build one let alone two and that tech lab cost already massively restricts what tech units can and cannot be run. More often then not I see mammoths these days crawling slowly from the base right as the last missile goes off to kill my opponent - building a mammoth pretty much guarantees loss of tempo unless you're far ahead, in which case why do you need to build a mammoth? :P
