OP - some Sheratons in the US are older (even some Westins) and can be more run down. Sheratons are nice for a 3.5 star brand (in the US) but I would hardly say they are Starwood's pinnacle. I recently stayed at the Sheraton Dallas Galleria, which is under renovation.
They put me in a newly renovated room. Cool! I did see one of the unrenovated rooms and floors, and it was like a completely different hotel. Which is probably why they are renovating it.
Marriott acquired Starwood mainly because Starwood has such a presence in the upper tier/luxury brands in major metropolitan areas. I would dare say you would think differently of Starwood if you spent a night in a St Regis/Luxury Collection/selected Ws.
I recently stayed at a W in the Middle East and it was arguably one of my worst Starwood experiences. Every hotel can have flaws, no matter how good.
Sorry you had 3 bad experiences with Sheratons - which, btw, you should contact management at the property about, as they are usually equipped to compensate guests with legitimate issuea well at Sheratons in the US - but you should give a nicer brand in their portfolio a try.