Combined arms is definately best. I like a mix of the heaviest down to the lightest troops. This gives you a good strong line of heavy infantry or pikes, some skirmishers and archers to drive off HA's and hit and retire types, some heavy cavalry to break heavy infantry lines (after engaging with your heavy infantry of course), or to drive off heavy cavalry, I also like having some skirmisher cavalry to pour missiles into enemy lines' flanks and sometimes add numbers to heavy cavalry charges.