As it happens, this site contains finished maps only. Yet, there are many maps around in beta stage. Some have been in such stage for years and most likely will remain so. I thought we could create a thread with your recommended maps still in beta stage.

To kick the conversation off, a map with a great gameplay and design, done by experienced mappers, is called "Hektik" (ws.q3df.org/map/hektik_b3/). It was last updated on 2013. Votes and downloads on the linked site suggests is a popular one.

Looking forward to see your favourite beta maps.

yeah, I like hektik too. another one I play on a lot is astronomybeta

Hey, that's a cool map! Added to my list! (it doesn't show in the main menu though)
Edited 10.58 minutes after the original posting.

The final version of Hektik is in quakelive iirc. I don't play quakelive.
This thread on esreality. might be of interest to some. (or just me :) )
www.esreality.com/...ik-for-quake-3/
I also found a 'final' version of Hektik on this site:
quakefour.net/maps/
Just browse down. I didn't want to direct link to the map...
Back to topic:
A forever beta is this map
ws.q3df.org/map/myth282/
It is a quite interesting one, a lot of work has already put into it by Castle and yet you can understand why he left it as it is.... A forever beta imo.

And my own map 'AntiBiotic' never got to that final release state either. I don't consider it as a 'best map in beta', that is Hektik... But for my map, there are just too many things 'meh' about it. Its size and item placement just to name the obvious two.
Still it might be fun with some kind of mod, like hunt. I just played it recently and I still like the curves and atmosphere of the map.
ws.q3df.org/map/jas3contest1/
Edited 55 seconds after the original posting.
Edited 10.43 hours after the original posting.