In theory it's:
Fairy Circle - breakpoint 26 - VP's: 4 3 2 - After the first time you play a minion here each turn, you may play an extra minion here or play an extra action.
Due it's ability it usually don' take so long to break when people invest on it (usually they don't due 1 VP difference).
In practice it's a tie between:
Wyrm's Desolation - breakpoint 20 - VP's: 5 3 2 - Each minion here has -1 power.
Innsmouth - breakpoint 25 - VP's: 5 3 2 - After each time a minion is played here, its owner may place one card from any player's discard pile on the bottom of their deck.
Antarctic Base - breakpoint 24 - VP's: 5 3 2 - Players cannot play more than one minion on this base each turn.
Wyrm's Desolation is great against swarm factions (even more so if you can move them there against their will.)
Innsmouth just has a big number and no offset mechanism.
Antarctic Base can't be swarmed (great for dino or apes tough)