I never saw Spock as the
main character, more of a co-main character in TOS with Kirk. But I think it can be argued that Spock is the glue that held TOS together more than Kirk did. The fact that Pike was replaced by Kirk proves that for me, at least.
As for the TOS movies, more or less follow the same pattern.
Now when you examine Star Trek as a whole, I think it could be argued Spock is a more important character than Kirk. Spock and his father appear in TNG. Spock is central to the reboot. Nimoy spans the 1960s, 70s, 80s, 90s, 2009, and 2012 as Spock.
Kirk finally does appear in TNG in generations, but Kirk's story comes to a close in 1994 with his death, at least until the 2009 reboot.
So I think to TOS and its movies, maybe Spock may be equally important as Kirk, but to Star Trek as a franchise, Spock is more important.