TV-7 D/U is a popular and well featured piece as well - nearly as good as the hickoks in a lot of ways. Generally cheaper, very ruggedly built, and easy to come by. Can test any tube for audio that you would be able to test on the best Hickoks.
The Hickok 539b/C is a very nice piece, however many people with more money than brains have driven the prices out of sight on Ebay. Most don't even know *why* it's a better tester, but they just heard it was and now they want it. The only really "better" thing about it is the facility for easy hook up of a plate current meter, something that can be done but is more work on most other testers. In the Hickok lineup the 580 is also and excellent tester. Rare and not often seen, it was a replacement for the 752 which itself was designed to replace the 539 series. the 580 is fully transistorized, no tubes inside to replace, so calibration is not really an issue as it is with the others. Also the easy ability to do a dual test with the push of one button is nice. (Dual triode sections can be tested with just one button, no resetting of dials needed.) There's much good info on the net, do a google.com search...
-Ed