When it comes to Indian buffets, my favorite one is whichever one I happen to be in at that particular moment. I don't have the most discriminating palate, and all I have to do is avoid certain lentil dishes (because me and lentils don't get along so well -- bad things can happen), so I just rotate around the ones I've found (and keep an eye out for new places).
Jason mentions Minerva in the Herndon Clocktower and Mayuri in Hunters Woods. I've been to Minerva and I like it -- it does get crowded, though. I'll have to give Mayuri a shot sometime (it's practically around the corner).
Also, one of his commenters mentions Banjara in Ashburn -- I think I've been there for lunch, if it's the same shopping center as Kirkpatrick's. Pretty good, and good desserts, as well as the little appetizer things. I forget what they're called, but they're good.
For my own part, I was just at Harvest of India in the K-Mart shopping center in Herndon on Sunday. Despite what I said earlier, it is one of my favorites. I started going there getting carryout vindaloo when I first moved into the area, and it's stood the test of time.
Also in Herndon is Supper Club of India in Worldgate; I've only been to once or twice, but it's also good.
Going in a little farther, I've eaten at Supper Club of India in Arlington, which I probably would have enjoyed more if I hadn't been hungover. And I do need to try out Tandoori Nights some time.
Other than that, I've been once to a place in Fox Mill shopping center -- I don't even know the name of it, it's next to the Five Guys. It was okay -- the only time I went, it was towards the end of the buffet, so it wasn't top quality stuff.
There used to be a Dehli Dhaba in the Herndon K-Mart plaza, but then it changed owners, and now it's a Five Guys. Or maybe it was the Afghani place next door that became the Five Guys. Either way, the Herndon K-Mart shopping plaza on Elden is still one of my favorite dining areas, since it's got Harvest of India, Charcoal Kabob (which I love for both kabobs and curries), a pho place, the Madina Market (where I get my Thai chiles and Afghan flatbread), a Thai place, and a few other different cuisines.
Like I said, when it comes to Indian buffets, I'm pretty forgiving. Though as I get older, I find that I can't power down as many plates as I used to (I think I'm down to 2 and half plates, plus dessert, though that does keep me fueled for the rest of the day, including the mandatory postprandial nap), so maybe I should focus a little more on quality.