REALLY good question. Clean records are far more important than one thinks.
Ultrasonic, BUT all ultrasonic units are not created equal, which is why I made my own.
How good is it? Going from VPI to my ultrasonic was MORE than going from good MM to higher end Koetsu. Hard to believe, but true - when I said that I could almost believe the choir was in the room, my wife (who sings in a choir), said, "No, better, because you can clearly hear the concert hall."
Things to watch out for: high frequency US (I use 80 KHz); enough room in the tank for ultrasonic waves to resonate, otherwise they will not clean properly (2 cm for 80 KHz, 4 cm for 40 KHz, though more is better); high power; good energy distribution; rated for continuous use. In short, a quality commercial unit adapted to record cleaning.
I should have spent the first big chunk of money on keeping my records extra clean, because 1. reduced stylus wear, save money, prevent record damage 2. big investment in sound. Instead, I did it last, and burned serious money on replacing cartridges, which I had regarded as expendables.