That was my first though as well,
Ika Ika Musume. Magearna is the slowest viable legend, and does not care about burn or intimidate. The ability is insane. With Steel Beam, it now has access to two 140 BP STAB moves in dynamax, as well as an additional nuke outside dynamax. Low BP moves was its main drawback last gen, which is why it almost always used a Z move. With dynamax, this problem is solved. It can even set up misty terrain to prevent amoongus from stopping it. Several of the top teams in a USUM online competition used it (almost same format, 4v4 doubles with legends), and at that time its worst nemesis Primal Groudon was widely used as well. I think Life Orb could be great on it, even if it chips away at its bulk - it is not
that powerful without a soul heart boost, so it needs all the power it can get.
Dusk mane is also ridicolously bulky, as theorymon said, and this bulk can be further increased with steel- and ground max moves. Weakness policy is a natural fit on it, all this considered. It is worth mentioning, though, that incineroar is problematic for it with intimidate and burning jealousy.
Another legend I think could be good is Solgaleo. It has slightly less bulk than Dusk Mane, but comes with great speed (97), better coverage (For example, powerful fighting and fire moves against the steel types that dusk mane struggles with), and most importantly: Full Metal Body. Being immune to intimidate could be huge here. I also thought of one specific combo I wanted to share: Max speed WP Solgaleo + a faster Bulldose user (edit: great minds think alike,
joey hassam!). Bulldose's speed drop takes effect immediately, but Solgaleo is immune to it. So you break sashes or shadow shield, slow down the opponents, and attack immediately after with a +2 Solgaleo. Perhaps something that can be built around?
(On a sidenote, I would be absolutely shocked if Incineroar is not the most used pokemon in this competition. Such a great matchup against many of the legends)