Laminator_X
Once & Future KMC Hawkeye
I say counter the prep with characters whose powers are undefinable or unpredictable.
My team:
Adam Warlock - Obvious I know, but he's got the most experience at Thanos-foiling and he's even got the soul of a Thanos clone trapped in soulworld for reference. Also, his powers are extremely hard to predict and thus counter (e.g. avoiding Galcatus blasts, devolving enemies, being invisible to Nebula, coming back from the dead and turning him to stone). Finally, Thanos fears him. Thanos's whole involvement in the Magus vs. Warlock conflict was just to avoid a direct one on one confrontation with either of them. He knows the Soul Gem could own him is Adam plays hardball. Surprise move Thanos won't expect: Cramming Warrior-Thanos-Clone's soul into Thanos's body for an instant hellaciously debilitating power struggle.
Johnny Thunder (New Version) - A tough but creative child backing a 5th Dimensional Imp with the mind of a vintage superhero The imp is such a wildcard that there's only so much prep you can do, and its powers are beyond the reach of Thanos's mere 3+time dimensional technology to limit. Surprise move Thanos won't expect: "Give that @^%#*!$&%#$ an atomic wedgie"
The Phantom Stranger Thanos (thanks to my pick) will be gunning for him personally so the Stranger will be free to act directly. With his foresight, he'll be able to warn the other heroes that Thanos is prepping to destroy them, and can watch over them to stymie any pre-emptive assasinations from the Titan before the fated battle can be joined. The vague/versatile/quasi-religeous nature of his powers make him hard find, let alone prep against, and Thanos targeting him negates his weakness, that he often may not act directly. His special mission will be to protect young Johnny, the lone vulnerable human in the bunch. He pushes the Skyfather limit, but no moreso than Dr. Strange. Who knows, he might just redeem the Mad Titan's soul? Surprise move Thanos won't expect: Breaking the forth wall to play mindgames with Thanos as Thanos reads his comics.
Scarlet Witch - (Busiek Era) Why? Because she makes the improbable certain, and when Thanos is involved you need all the luck you can get. She might even shake off a mutant-nullifier attack if it didn't catch her flat-footed. Bonus points for conjuring/ressurrecting Wonder Man in Thanos' face (if that wouldn't put me over the team member count.) Surprise move Thanos won't expect: Making him overlook something crucial that bites him later in the fight, He'll be ready for direct attacks, but he's too arrogant to entertain this possability.
Gamora - His adopted daughter is just the emotional Gadfly to keep Thanos on edge in the heat of battle. Since her ressurrection she's no longer amped up by implants that Thanos could just shut down She's literally fought him to a standstill in single close-combat, art prevailing where bricks fail No real weaknesses to exploit, and if Warlock goes down, get ready for bloodlust. Surprise move Thanos won't expect: "You know 'Father,' when Adam recreated our forms, he removed the failsafes you placed within my dagger..." Did he or is she bluffing? I don't know and niether does Thanos.
None of my crew is easy prey for "Bat Shark-Repellant" type prep or have glaring Achilles's Heels, and the Stranger means Thanos won't have surprise or perhaps even the choice of battleground.
Characters under the powercap who can match Thanos power for power are few and far between. These non-linear types are tough to prep against short of preemptive assassination and don't necessarily need to overpower Thanos to defeat him, although the Imp has the juice to take him on at least.