I agree about Minllyn. Considering that most of the chambers are just one working floor high, they have a scale and grandeur. When taking groups in there, it's good to stop in the first chamber, turn out the lamps and look up to the daylight from the top entrance. The flooded chamber also impresses, especially the younger groups I go in there with.
It's interesting that folk have mainly, mentioned chambers on the steeply-dipping Narrow Vein. Some of the Cwmorthin chambers and the ones mentioned at Moel Ffrena, must be just as long. I suppose that they're less steep and visually less spectacular.
A couple of chambers in the Gaewern/Braich Goch complex go 4 floors down (5 to 8 Sgwd) and then another 3 (Vanes, 5 & 6). It was a spectacular place to be able to take people for their first underground experience.