I think Liz is the current top tank overall, but she’s only being tested as possible new meta at the very upper echelons because she’s still relatively rare compared to many of the various purple options, and the other best red tanks (BK, costume GM, maybe Octros, maybe Garnet) are all incredibly rare too, so not a lot of alliances can viably field red tanks.
IMO, Bera’s still the best purple tank. She punishes bad boards by stacking up those nasty poisonous minions, and she also screws up what otherwise would have been good-enough boards a lot of the time by cutting your mana after you make your last on-color match. (I mean, ideally, you’d work around her mana cut and save a match for after she’s dead, but that’s not always possible, and cascades often make it happen beyond your control.)
Freya has the mana cut, and punishes bad boards similarly with her minions, but it takes longer for those minions to really dig in and damage the attacking heroes, because they don’t start doing serious damage until there are three of them. Bera’s minions also keep damaging you after they’re dead, via their poison; if you use a minion remover like Grimble or cGormek, Freya’s minions are fully neutralized immediately.
Moreau is just kinda meh as a tank, imo. He doesn’t punish bad boards, and it’s easy enough to work around his ailment (or cleanse it) if need be.
Hannah and Xnolphod can both punish a bad board by stacking up fiends and speeding up flank/wing specials, respectively. But both die to three mono yellow tiles or six mono neutral tiles, and don’t cause any problems for the attacker when they do so, like Bera and Freya do. (Also, if Bera/Freya fire early enough, they would take more than 3/6 tiles to kill b/c of the extra minion health. Hannah has this going for her too, sometimes, because she weakens base tile damage, obviously.)
I don’t think purple LBs next month will change any of this.