Maxwells is a definite. I've also heard good things about Tarkopia (sp is wrong, it's a blue pharm book), and Sanford's abx guide; I purchased both books but have been relying entirely on my palm programs for rx.
Also-- Medicine on Call is great when you're (what else?) on call. I didn't like Ferri quite as much, and Wash Manual is rather substantial. My personal fave happens to be the Oxford Handbook of Clinical Medicine but unfortunately it hasn't been updated recently.