Best South Indian in DC ???????
You'll find nothing exclusively South Indian in DC. Only in the deep suburbs where that population lives. Like Saravana Palace in Fairfax, VA or Woodlands in Langley Park, MD. Both are impressive.
Your best bet to get something non-Euro in DC is probably Bombay Club, a fancy place that will turn up the heat if requested. If you're out for a night of more delicate spices, than Rasika, their sister restaurant, is a gem.
If you don't mind taking the metro out to Bethesda, then Passage to India has a South Indian section to the menu that is very interesting. If you go, make sure to order the pickles and the spices from the last page. This is a your best bet for a restaurant that dabbles.
There is a Woodlands in Langley Park as well. Their brunches on the weekends are decent. Unfortunately, the best place that I would recommend has sort of closed. It was called Udupi Palace and it was in Langley Park, near the intersection of University and New Hampshire. It was a strictly South Indian vegetarian place and was pretty good. I say it "sort of closed" because it merged with a restaurant a block away called Tiffin.
They claim to have merged the menus and said that they make everything Udupi made (making both Northern and Southern Indian food, which already makes me nervous). I asked if I could have the South Indian Thali I used to get and they told me they didn't make it. There was one thing no longer on the menu. I left in disgust and went to Woodlands. But, it might be worth a try.