For in-game playthrough, I would recommend Lapras or snorlax
Lapras has decent bulk and overall great stats. It's G-max resonance is super powerful and can reduce the damage your Pokémon receive for five turns. Also Lapras has an amazing movepool so you can use it to your advantage too.
Snorlax is very bulky and has great coverage with zen headbutt, high Horsepower, etc.
With good stats, including a massive 160 base HP, Snorlax is going to be sticking around on the field for a solid amount of time. Using the time spent Gigantimaxed to set up might not be a bad idea, but even if you just want a solid tank with lots of options, this is the safest pick in the game.
G-Max Replenish also automatically recycles berries, meaning Snorlax can just keep munching on whatever effect berry you want to give it.