Batman, Wonder Woman and maybe the rest of the JLA.
The Kents, Pete Ross and Lana Lang.
Only few persons for a big secret!
I don't know about Lana at all - one of the more interesting subplots with the character was in the eighties Lana started to get hot and heavy with Superman, without knowing he was Clark Kent. Keeping an identity secret while doing a romance is a much more interesting idea than having a romance and letting the girlfriend know everythihg. Why is it every romance in a comic book movie ends with the girl knowing the good guy's secret, anyway?
But beyond that, it makes sense that a guy like Superman would need Clark for his life to have some sort of normal dimension: he is not only the greatest celebrity on earth, he is its most extraordinary citizen; he'd be mobbed all the time without Clark and would be forced to live in the Fortress of Solitude all the time, and would end up like Elvis in Graceland.
While I'd never thought of it that way, it IS an interesting idea:
One of the reasons Superman needs Clark Kent psychologically is that Superman is too humble to be "Superman" all the time.
I don't think that Jimmy should be in on Superman's secret. Superman probably thinks of him as mostly a part of Clark's life, a part of his normal life, and he would like to keep his two identities completely separate. The fact that they sometimes coincide is probably something he regrets, but was probably also unavoidable after a while. Ideally, Clark's friends would never meet Superman, and vice-versa, but it's like the work-life balance, I suppose. Sometimes it spills over, no matter how hard you try.
This brings up an interesting question, which is why and how it became known that Superman "made Clark Kent his friend." It's a common wisdom done by everybody from Morgan Edge to Lex Luthor on up that Clark Kent is an old friend of Superman's. Why would Superman do that, I wonder?