Many of them can be made into respectable runners. The 283, 305, 307, 327, 350 and the 400. However on a popularity scale the 350 is the most popular and parts for it are the cheapest. One thing to keep in mind is the small cubic inchers such as the 283 and the 305 are somewhat more difficult to wring power out of due to there small cubic inches. The 307 is likely the most scorned small block ever but it can easily be made into a strong runner.
If you want a play toy engine then get a 350. As said, good cubic inches, parts common as the day is long, parts cheaper, and just like the rest of the small block family easy to build and easy to hot rod. You don't have to spend alot of money to get 300 horses out of a 350.