I don't know about the firmest, especially since I haven't tried them all, but...
With tenth frame (being all that my local shop carries) I had to replace inserts every other month or so, bowling two nights a week mostly with the same ball.
Since I started ordering Vise inserts online, I haven't had to change any inserts at all in about 9 months. Specifically I use clear powerlift/oval on the powerlift side.
Purely anecdotal but there's my $.02