In terms of acoustics, equipment and service, the best place I've ever been to is Backbeat Studios...they really deliver magnificently in all those areas. However, because I'm just a lowly paid intern, I usually settle for Beat Merchants...which is good also, but slightly inconsistent from room to room. I usually prefer Room 5 as it's on the ground floor=privacy. Plus, they use it for recording so it usually has the best equipment.
The absolute worst would have been Tiet @ Outram area. Equipment fairly decent, but the prices were outrageous last I jammed there. Acoustics also nothing to write home about, and size of the studio...fooyoh. Also I had a bad experience once where my band booked a slot to jam and ended up waiting outside for 2 hours because there was a power cut and no one bothered informing us. Gave us a free session back, but damage was done. Wouldn't jam there again now even if they offered us 10 free sessions.