Difficult question having used so many different rubbers in all the years
Back in speed-glueing times i really liked the Stiga IUL and later Neos Sound, speedglued really good spin and speed and a nice touch.. Now after the speed glue ban on the backhand i play Tibhar Nimbus, tested one pre-series sample and still stick to this rubber since then. So it's about 4 years now with Nimbus and i still like it

On Fh i tried so many rubbers, Nimbus, Sonex JP GOld, Sriver G3, Hexer Powersponge... All of them were quite good but so often durability is the main problem.. Atm I'm using the Hexer Powersponge and have used 4 or 5 of it and it's quite good..
So for an "all time favorite" i probably have to divide into speedglued and none

With speedglue it would probably be Stiga IUL although it was nearly a bit too soft glued and Bryce with massive glue was really nice, too. Withouth speedglue it probably would be Tibhar Nimbus.