After the release of sun and moon and few attempts at balancing tiers we finally made it to Starmie and as such it is time to look at the pro and cons of Starmie so we can evaluate its true worth.
First let start off with its stats. Defensively it is 60/85/85 which isn’t great, but it also horrible as that is enough to often take a neutral hit on about any starmie set as this calc shows.
252+ Atk Landorus-Therian Earthquake vs. 0 HP / 4 Def Starmie: 222-262 (85 - 100.3%) -- 6.3% chance to OHKO
The attack stat isn’t why we are here since it does nothing useful at 75. 100 Special attack is decent for offensive mon depending on their typing and stab(s) with which starmie is fortunately having such a stab coupled with a great 115 speed which allows it you outspeed a good portion of legal mons to make use of its offense and utility.
On to the typing it is a blessing and a curse. It has a total of 5 weaknesses with one of them being dark so it can be pursuit trapped which is annoying thing when preping. It typing gives it resists to fighting, steel, water, psychic, fire and ice which is overall good considering more of those then not are useful stab or coverage move types and contains the 2 most common hidden powers so it has the opportunities to switch in and the most of its defensive stats.
Next it has 3 abilities with only 2 having any competitive use, but both being useful abilities. Nature cure is a useful ability allowing it to remove any status when it swaps out. Its primary useful on defensive sets so you don’t get worn down by toxic or burn and to be able to provide speedy utility throughout a game. It also has some use on offensive sets as it allows Starmie to remove paralysis, which is something you never want on an offensive starmie since it seriously hampers Starmie ability to do damage. Starmies other competitive viable is analytic, which makes starmie do 30% more damage when it moves last that turn. It sounds not useful at first until you realize that switching gives the analytic boost giving Starmie a free life orb on offensive sets supporting its coverage well.

Now on to the notable moves it learns.
Dazzling Gleam
Flash Cannon
Grass Knot
Hydro Pump
Ice Beam
Light Screen
Magic Coat
Power Gem
Psychic
Psyshock
Rapid Spin
Recover
Reflect
Reflect Type
Scald
Signal Beam
Thunderbolt
Thunder Wave
Trick

The first that may stand out when you look at the pool of notable moves is rapid spin. Starmie is the fast natural rapid spinner in the format and arguably the best rapid spinner with only Excadrill as competition. To compare the 2 tier 1 rapid spinners, Exadrill is the more powerful of the two while starmie is the more versatile of the two. Both of them primarily are offensive rapid spinners, but can have defensive uses which starmie does better despite the more weaknesses due to reliable recovery and a large pool of utility moves to make use outside spin compared to Excadrill much smaller utility move pool of stealth rocks and toxic to have some uses with rapid spin. On the offensive time Excadrill is the more powerful of the 2 with a higher offensive stat and the ability to boost with swords dance. However Starmie is the more self-sufficient offensive mon of the 2 for one reason. Excadrill sits at base 88 speed, relying on sand or a choice scarf often when trying to make the most of its offensive presence. Starmie having a lower, but decent offensive stat of 100 can have a lot of offensive presence with an offensive item and analytic without requiring a certain partner or wasting turn to set up weather. Starmie also has a somewhat easier time fitting into drafts due to being more self-sufficient as Excadrill partners for setting up sand are either rock or ground, sharing a weakness with Excadrill.
Now after comparing it the other rapid spinner in the tier I will briefly compare it to the defog users as despite rapid spin being better then defog, defog has a lot better distribution and uses compared to rapid spin. The users are Zapdos, Skarmory, Mew, Latios, Latias. Compared to 5 the most notable thing is, Starmie is the fastest mon of the group with more than half of them sharing the psychic type with starmie with the annoying weaknesses to pursuit trapping. Starmie is on the lower end of offensive and defensive stats, but is the better side of utility among the 6. The main niche that I would say starmie has among the defoggers and even drill is overall, outside of mew starmie is the most versatile hazard removal in terms of offense and utility while also providing a bit of speed to a draft.

After comparing it to the other hazard removal in its tier, I would say Starmie is in the proper tier do to the mix of offensive and defensive compared to them. It has a bit of utility above many of them while being a bit lower offensively then them but not a ton lower due its coverage + analytic.