None of them. Not even RTM.

In comparison to current Win 10 (aka Windows - neverending beta edition), any older beta is stable and worth using. Well, at least to me. 😉
Edit: Not very stable but (IMHO) interesting are Windows 2k Beta (Alpha/PC-98), Neptune, Whistler (pre-Beta2) and Windows Longhorn with the Plex design.

Isn't the PC-98 just a series of x86 computers?

Yes, a japanese model. Windows 2000 (beta ?) is notable for beeing the last Windows it got.

I don't know which ones are the best or the most usable, but there used to be a forum dedicated to beta OS's (something like betaarchive).
It had a lot of knowledge contained within its community and they traded beta disks kinda like they trade om Amibay.
Dunno if it exists, but if it does, it's probably easy to find using google.

Personally I've (at the least) used a beta of Windows 98 and I've used Neptune, but both for only very short amounts of time.
The Win2k beta3 seems to me the most interesting one, as it supposedly had support for hardware that the release version of 2k didn't have anymore.

If you get bored of the beta OS installs you could always track down a copy of the Windows 95 floppy version and spend a day installing that for giggles.

Memory plays tricks on me, but I recall using a Win95 beta for a while that was on maybe 7-10 floppies, and it seemed to multitask better than release. You could do other things while formatting a floppy, for example.

I used it well after release, until something came along that wouldn't work anymore.

This was in 1994, so it had to be earlier than build 300.