I'm pretty sure that most members here have adopted RMGI 468 as the default when it comes to currently available tape for long term storage and great sound. The Tape Project tapes are recorded on this tape and our director Paul Stubblebine has gone into the reasons why he likes it elsewhere.
But, if I were making live recordings with their higher demands on dynamic range and absolutely the lowest noise floor available, I think I'd give the new ATR Magnetics tape a try. From what Kurt over on US Recording Media has indicated, many studios have adopted this tape and are very happy with it.
That said, I've have yet to try it myself and I don't know what you might be looking for in sound.
I moved this post in hopes that those among us who
have tried it will chime in, particularly on how the ATR tape sounds with various machines/heads.
There are links to the data sheets of all the currently available tapes here;
http://usrecordingmedia-store.stores.yahoo.net/BTW Welcome to the forum!