Nobody has added a response to this thread for 5 years. But it still comes up on Google as the first search result for "Best Reel Tape". So how about the latest ideas about which tapes are worth searching for, either used on Ebay or current production? I believe Pyral (formally RMGI) is the only new option now. My first advice is BUY NEW from a Pyral dealer, to support the continuation of new production. Their 468, 911 and 900 are generally considered the best there is.
My second advice is before you chose any tape, you must decide how hot a tape is appropriate for your machine. Then within each range, there are a few "best" tapes to look for. There is no reason to settle on just one tape in particular, unless your machine must be opened up to adjust bias, sensitivity and EQ. For Otari owners (me for example), and for many other semi-professional decks, we have test-tones and external adjustments to adjust and optimize for any tape within the limits of our decks' performance.
For the 0dB applications: Maxell UD, UD-XL, UD-XL1, TDK LX and TDK GX are great and plentiful. For +3 dB tape: BASF/RMGI/Pyral 911 or Quantegy 456 (not Ampex 456 though). For +6 dB tape: Ampex/Quantegy GP9, Quantegy 499 (not Ampex), ATR Master, BASF/RMGI/Pyral 900, Agfa/BASF/Emtec/RMGI 468 are all great. You cannot go wrong with Maxell, TDK, and ANY manufacturer of 900, 911, GP9 or 468, because these formulations NEVER had a widespread problem with sticky-shed. And, you can never go wrong with ANY non-backcoated tape from the past. If the back and front are the same color, it will play fine. If the backing is black, you MUST know exactly what tape it is before you subject your valuable tape machine's heads to that tape.
Again, if the backing of the tape is black, and you don't know EXACTLY what tape it is (and what year of manfacture) then you should remove the tape and throw it away. Keep the empty reels for new tape.
Ok, how did I do? Did I miss anything?