If you want a classic lens, there are plenty of options, but you have a classic lens, for better or worse. (Image quality, coating, etc.) How vintage do you want to be? Lotta good stuff out there in thread mount pretty cheaply.
New color-skopar 2.5s are getting scarce and expensive. B&H seemed to be out of them at the rock-bottom $289 price, but other places (check ebay, etc) are $3-400ish (photovillage, plus there was an independent seller recently), and CameraQuest (not gray market, and apparently the only ones in the US despite claims to the contrary) is $599. I just picked up a B&H before they were out of stock. Hope it doesn't come with the focusing problems of my 21mm CV, because I got jerked around on that. B&H told me to go to the manufacturer who told me to go to their US distributor CameraQuest who told me my lens was gray-market despite what B&H said. Eventually I badgered B&H into a grudging exchange.
So you're going to need to define "best." Smallest? Best value? Widest aperture? Easiest to carry? Sharpest? Classic look? I went with the Skopar after determining my needs were for the most compact lens with the easiest handling (eliminating collapsibles like the 50 3.5 Heliar and Elmar-M 2.8). It was a tad heavier than I wanted despite its size, but I couldn't argue with the price, and I will wager that unless I get a bad sample, it'll be as sharp a lens as I'll ever personally need.
If you don't mind the relative size and weight (still tiny by SLR standards), you likely could not go wrong with a Summicron in good condition. Tough to find in your price range, however. But they'll hold re-sale value in the end.
