Depends if you're talking visual or instrument flight. In VMC then yes it would be the altimeter as you would be setting the pitch attitude using the visual picture - the AH should be largely redundant. In IMC then of course the artificial horizon/attitude indicator becomes the primary pitch reference and the altimeter becomes a performance (or confirmatory) instrument. Many VFR-only aircraft don't even have artificial horizons.
[ 20 October 2001: Message edited by: DB6 ]