I assume that with "reliability" you mean the duration in time of a burned disc
Media reliability is determined first of all by the media quality itself. A bad quality disc will give bad results whatever is the burning speed.
Because of there are variations among production batches, it is impossible to answer your question. Some excellent batches of 8x media can give excellent results even if burned at 18x (not so frequent, but possible).
As a thumb rule, however, very often 16x certified discs give best results when burned at 12x or 8x. There is a general agreement that when a disc contain a lot of errors after the burning, this disc will have a higher probability to become unreadable after some time. So, if you find a proper speed to obtain as less errors as possible on a disc, this is the most probable way to obtain a reliable burn.
In my personal experience (i.e. with the batches I was able to get from stores), TY 16x certified discs give the best results when burned at 12x, or also at 8x (results are basically equivalent in my personal experience, so I prefer to burn at 12x because less time is needed).
I hope I answered your question, sorry if I misunderstood it