Cindy Huntress when it comes to sessions. She's so powerful that she can get an instant tap but the toughest part about her scissors is that she just engulfs you with those legs and creates an almost smothering effect. Inescapable and you're just there until she wants to let you out. Jen Thomas has a similar set of scissors, IMO.
As far as the best I've ever encountered, believe it or not, it was an old girlfriend and beauty pageant winner. At over 5-10 and blessed with long, lean but still thick and muscular legs, this girl was built for scissoring. On more than one occasion, she put me out but I can say with 100% certainty that I've never felt anything like her figure four (either direction) or her straight on scissors...especially when facing her. I'd say those and Cindy Huntress' reverse scissors were as overwhelming a hold as I've been placed in.
In the case of the pageant queen, she had a way of pulsating her inner thigh muscles while also squeezing at full power that literally weakened me by the second, to the point that you felt as if you were in some sort of trance, unable to so much as move or even think. And with legs as long and as strong as hers, forget about getting out. For those of you who REALLY know about scissors, you'll understand this: Even her feet were overpoweringly strong, as in you couldn't come close to prying them apart with your hands. Unreal. There was literally a point in time during one particular match that I remember realizing (during a figure-four) that I was literally in awe of her strength and ability to scissor.
...and I must have caught Milady on a good (or bad) day or whatever, because her reverse scissors actually scared me. For some reason I didn't tap even though my brain was telling me to do it and she just kept squeezing harder and harder until I thought my head would explode. Nothing but respect for her after that session because it ranks as a top three, all-time.