It depends on who you're insulting and why, I suppose. I'd use 'git', but only 'dolt' when I was being sarcastic; I've never really heard it used elsewhere. 'Idiot', 'loser', 'twit', 'sod' are all words you could use. My dad uses 'wally' when one of us are being stupid, but that's only on humourous occasions. They're all I can really think of at the moment.