Are you looking at an Nvidia Card or ATI? It would swing my answer.
For Nvidia, my favorite brand is XFX hands down. Although Evga is good and several others. MSI is nice, whoever has the better cooler and better support.
ATI, Sapphire is the largest seller. Some people don't like them, however I have never had an issue. Asus is nice, I have an asus, their support is good and quick. HIS probably makes the BEST coolers, but you will pay for it, which sometimes isn't a bad thing because if you plan to upgrade your cooler you may as well go with one of their IceQ coolers.
I like EVGA, XFX and BFG. Always check price, then check each of their websites to see about warranties, support and upgrades. Another thing to look for are good cooling solutions and whether it exhausts hot air out of the case or not (more important on cases that don't have good cooling themselves) Because of recent price drops it may be a better buy to get a 9800GTX+
Anyone remember Pine technology I've known people at work that bought them and had lots of problems, anyway thats where XFX came from I stayed away ever since then. I know people that use them now with no problem but heard some bad stuff about thier motherboards though.
I like cards that come with FREE GAMES!... That is always my selling point. Unfortunately this time they did not, although if I would have waiting 2 weeks I would have gotten STALKER Clear Sky, I was upset about that.
I think they all generally keep to the same reference design, so the cards themselves, all things being equal, should be similar in capability and reliability. What you want to go for are the warranties, extra goodies such as free games, upgrade programs, and the quality of their technical support. All things considered, I like EVGA.
Well I like XFX, No one that I personally know (online buddies, gf, father, brother, friends, friends' rents, etc) have nvr had any problems with them, They are not the best for performance (someone else always has better oc) and are not the cheapest (always cheaper somewhere else lol) but they are very nice and reliable which is what I care about :-D