This happens often when I try to join a server.. I've patched my version to 451 but I'm not sure if it would have been better to stay with the 436. Why is some servers rejecting me? And which version is best to version?

Having wrong system files will get you a "package mismatch" message or something like that instead, but who knows with 451-that's actually a server-side patch. Clients should stay on 436.
Usually, a "Rejected by server" message means one of the following:
-The server's player/spectator slots are full (especially spectator slots, if you're set as one in the player options)
-You're banned from the server (highly unlikely if you've never been on that server before, but you never know)
Anyway, try rolling back to 436 and see what happens.

Lol! I thought 451 was essential to play online. Anyway, I've downgraded to 436 and it works perfect now! I found out that all servers running 436 will reject players with version 451 and that's why so many servers wouldn't let me in!
Thanks!!