Hmm, let's go through them, shall we? (can't ya tell I'm bored...)
S1 is
definitely a Sam season because Jared was number one on the call sheet for that very reason; probably explains why S1 Dean exudes secondary funny guy character. S2 arguable continues this thread, since it is all about other kids like Sam and his figuring out what it means to have powers and whether or not he will turn evil. Then, S3, definitely more Dean-heavy, as it is all about trying to keep him from going to hell at the end.
S4 and S5 are surprisingly equal, IMO, with Dean's arc being heavily intertwined with the Angel arc, while Sam's is all about Ruby and Lucifer.
I would actually argue
against S6 being a Sam season; yes, he has the whole soulless bit at the beginning, but I find it very Dean-heavy - Dean is the one having to deal with his brother having no soul, having to juggle that and his past normalcy with Lisa and Ben, having to deal with mistrust of the Campbell side of his surprise family, dealing with how odd his best buddy angel friend is acting, just feeling surrounded by enemies and finally given the twist that Castiel has 'turned'. I consider this a Dean season.
S7 is fairly up in the air between being equal and being Sam centric. 8 is fairly equal, from the first half where they both get dedicated flashbacks, to the second half, when Sam starts the trials and Dean has man-pain about it
It's with S9 and 10 that I think it's starts shifting and becomes
way more the Dean Show, than the Winchester show (not that there's a problem with that, lol). Dean gets the Mark of Cain, Dean becomes a demon, then it's S11 and Dean has that 'connection' with Amara, and then in S12, the show pretends, with Mary returning, that it's back to being about both brothers, but they acknowledge in early S13 that they messed up and made S12 all about Dean's man-pain...again.
S13 is my
favorite version of Sam Winchester (before that he barely made it into my top ten characters, now he's third place) so I may be biased to say that it's his season to shine. An argument could be made for it being equal again.
Can't remember the last two seasons enough to answer that and still haven't seen the last seven episodes of S15 (yes, I haven't watched the finale yet, I'm aware!)