Let's take a look at their movepools and stats, along with typing.
Pyukumuku-
Pros-
Nice only water typing to give it 2 weaknesses, Grass and Electric.
130 base defenses allow it to soak up hits nicely
Has access to Recover, Toxic, and Safeguard. Safeguard helps protect from any passive damage. Or you could just use Taunt.
Access to Soak to ensure all targets can be poisoned.
Unaware prevents being set up on.
Cons-
That base 5 Speed isn't helping anything
Lack of any Offensive presence at all. It relies on passive damage and has no access to any attacking moves except for Counter and Mirror Coat.
Total Taunt Bait.
Poor Defensive typing. Resists only Fire, Ice, Water and Steel.
Totally walled by Steel and Poison types.
Toxapex
Pros-
Great combo of 152/142 Defenses.
Has only 3 weaknesses and 8 resistances. Resists common threats.
Has access to Toxic, Recover, Baneful Bunker and Haze to stall nicely.
Access to Scald with a chance to burn for those pesky Steel and Poison types.
Regenerator provides some nice healing when switching out.
Isn't affected by Toxic
Cons-
Weaknesses to Ground, Psychic, and Electric are very common coverage moves on pokémon.
Again, it has no offensive presence with 63/53 stats.
Has the lowest HP stat of the 3 At 50.
Again, Taunt wrecks this thing, but not as badly as Pyukumuku.
35 Speed doesn't help much.
Milotic-
Pros-
Water typing gives it 2 weaknesses
95 HP and 125 SpD are good.
Access to Refresh which cures status conditions.
Access to Haze to prevent being set up on.
Also gets Recover and Toxic to stall.
Scald gives it a slight offensive presence. (Not including Competitive)
Cons-
Again, it's Electric Weakness and Grass are fairly common.
79 Defense prevents it from walling any physical sweepers.
Again, has a huge weak spot to Taunt.
Lackluster movepools give it nothing to counter Electric Types.
Even though Marvel Scale boosts Defense, you still get stalled by passive damage.
Low Speed again is a major problem given its low defense.
Conclusion
Overall, it is apparent that Toxapex is overall the best staller. It defensive typing allows for it to stall and use Recover, unlike Pyukumuku. Although it's weaknesses are common, it's defenses well than make up for it unlike Milotic who has to switch out when it just sees a physical sweeper. It's ability of Regenerator greatly surpass Marvel Scale and Unaware in its basic utility. Toxapex's pros greatly outweigh its cons as well. One more point, Milotic and Pyukumuku have easily exploited weaknesses while Toxapex's take longer to exploit. I probably missed a point of 2 as well. Hope I helped!
