The fact that it bears Spiritomb and Sableye? Or the fact it can spin + provide consistent offensive pressure + while having longevity (which is not true! ) Unfortunately Starmie does not have 5 moveslots, and it cant run all the offensive moves it wants, you either not run TBolt and get walled by by all the waters, or you dont run Ice Beam and become a free switch in for one of the best offensive breakers in the metagame (Latios) and a few other miscellaneous things (Alongside not forcing out stuff like Dragonite immedietely).
Fair enough though IMO I still think Starmie is the best or second best mon in the meta ATM.
It's a spinner that doesn't lose to a single spinblocker (outside of Shedninja who loses to hazards). In a metagame where spikes and SR are extremely strong. Which IMO makes spinning stronger than fogging as fogging removes hazards from the other side as well as your own.
It has an absolutely deadly speed tier. Being faster than every non scarf OU mon except Kazam, Azelf and Aerodactly. And with rapid spins +1 to speed it can then outspeed everything else in the game easily.
While true it has 4mss. It also can run a variety of sets. To defensive spinner, with recover + Spin + 2 moves with nat cure or a more offensive version of this with Analytic
It can run LO Analytic spin + 3 attacks of which Hydro + Bolt Beam is the best and its near unsswitchable into. (The only mon that really can is Gastro.) With this set it's also a good revenge killer due to its crazy speed tier.
And if you are running fog, it can just straight up run a LO 4 attacks moveset and be an incredible wall breaker on its own freeing up room for another mon to clean up and a final note is it's resistant to the 2 most common forms of priority in the tier in BP and Mach Punch which is a bonus.
I'm not saying Starmie is broken. It's not, it has flaws such as suffering from 4MSS and being relatively frail. However it's mix of utility both support wise, offensively and defensively make it the best mom in the tier IMO. Except maybe Scizor.