Thats like asking what kind of canoe is best on a river.
Alot depends on your system. AH is pretty CPU dependent, so even a mid-range video card probably wont get maxxed out if you have a slower computer. Good bet is either the ATI 9600 series cards, or Geforce 5700 series. Those are not the bottom of the barrel, but they arent the top either. If you want the mid-range stuff, although I'm very much pro-Nvidia, I'd have to recommend the ATI 9600 series as the best of the bunch. Pains me to say it, but its true. Really though, its going to depend on how old the system is, as to how much money you are going to want to sink into a video card. A really slow computer is going to suck in AH anyway, no sense in spending good money on a mid range card if you have to upgrade everything else.
As with everything, there are good and bad cards on both sides, so its best to research if you are going to buy. Check reviews (and not the ones shown to you by the retailer or the manufacturer). Check out Tom's Hardware
http://www.tomshardware.com for info, also try
http://www.nvnews.net for info on Geforce cards, and try
http://3dgpu.comalthough alot of their stuff is dated its still educational.
You can also look at
http://www.d-silence.comand even
http://www.driverheaven.netfor information on what issues different drivers are having with which cards.