It really depends on your setup. Like Authorize.net is fabulous, but it can be exhaustively taxing if you do not sell a tremendous amount of product.
I personally use Concord/EFSnet for my CC, Check and Debit authorizations. They allow me card-not-present, card present and check authorizations and processing, Online and Offline processings, remote processing via secure Internet site, etc. This makes it exceptional for visiting a client in another town. I simply use my pocketPC to connect and authorize their wholesale purchase on the spot. Very professional, gets them everytime, and inexpensive.
http://www.concordefsnet.com/
Anyway, pretty much every major "TRUE" merchant account provider (i.e. not PayPal, CCBill, etc.) will provide services as described above. Onje thing to really look for, Concord is THE backbone for many providers, so make sure they are not rerouting to Concord. If so, skip then, go with Concord, and save time and money! :-P
Good luck
Ruhl