I think most spaghettis are very well cast, this is nice, and makes the characters credible. This is something to be aknowledged!
There are both many regulars who turn up many times, Franco Nero, Tomas Milian, Eduardo Fajardo, Fernando Sancho, Klaus Kinski etc, and of course people like Eastwood and Van Cleef. Everybody great!
Then there are some who have only done one, or a couple of spaghettis, like Tony Musante or Johnny Hallyday. They're also great!
I can't think of a really bad cast in any spaghettiwestern, there are some actors that may not be perfect, like Giuliano Gemma, but he is still ok.