Starmie isn't A rank, it's supposed to be an offensive spinner and it's special attack is a low 299 and the only thing that's good about it is it's speed, and losing to both Scarf Gene and LO Torn-T do NOT make it A-worthy in my humble opinion. It was better when it was faster than Torn-I
...you're implying that Starmie is moved down to B-Tier because it loses to two pkmn when it comes to speed? One of which has to be scarfed? That's not really a fair statement IMO. Starmie is base 115, which outspeeds a plethora of pkmn in UU, only loses to Jolteon, Tornadus-T, Alakazam, and Dugtrio. Past these four, you need a choice scarf to outrun Starmie. Starmie's "low" 299 Special Attack easily gets bosted by Life Orb, and the fact that Starmie posesses all the coverage options it could possibly want in Hydro Pump, Ice Beam, and Thunderbolt.
I think the main thing you're overlooking is that Starmie is probably A-tier under support. It's a spinner. It's not sweeping through big portions of the metagame. It spins on whatever it wants to. The only two spinblockers in the tier (Jellicent and Gengar) can't stop Starmie. Jellicent fears Life Orb-boosted Thunderbolt, if not Thunder on a Rain Team, and Gengar is outsped, and can't take a Hydro Pump. In addition to the spinblockers, it provides significant offensive pressure in general. It's the only pokemon in the tier that can use Rapid Spin while not completely nuking any momentum you've spent trying to built up. Forretress can somewhat do this with Volt Switch, but it doesn't have the same coverage Starmie has, so a wider variety of threats, offensive and defensive alike, can come in as Forry uses Rapid Spin. Starmie keeps pressure down on your opponent's team, as only a few defensive walls (Blissey, Chansey, Ferrothorn) can safely switch into all of your coverage options, and none of them can prevent Starmie from spinning.