It depends very much on the build, wealth, and even playing ability. There has been a ton of theorycrafting about it, much of with can be found in the 99 theorycrafting thread and latest untwinked 99ers thread.
If you want the highlights, or at least good places to start figuring it out, here are some helpful links:
My general post-90 leveling summary
Gripp's epic sub-98 research and post
Nanomist's XP tables
That should provide you with more than enough information to start your research, but IMO, as far as we can tell right now the absolute fastest way to 99 would be (for a build that can handle these areas):
1. Pits/AT until you're capable of running CS (usually ~94/95 ish)
2. CS runs (Vizier/Infector seals) until 98
3. Nihlathak runs + Ancients to 99
This is not a blanket approach though, as for some characters it's faster to take other approaches (Baaling for a trapper or javazon, for example). But for the absolute fastest way to 99, with an appropriate build, this would be it.
If you want the highlights, or at least good places to start figuring it out, here are some helpful links:
My general post-90 leveling summary
Gripp's epic sub-98 research and post
Nanomist's XP tables
That should provide you with more than enough information to start your research, but IMO, as far as we can tell right now the absolute fastest way to 99 would be (for a build that can handle these areas):
1. Pits/AT until you're capable of running CS (usually ~94/95 ish)
2. CS runs (Vizier/Infector seals) until 98
3. Nihlathak runs + Ancients to 99
This is not a blanket approach though, as for some characters it's faster to take other approaches (Baaling for a trapper or javazon, for example). But for the absolute fastest way to 99, with an appropriate build, this would be it.