Well, visually, one of my favorite illusions from BeLIEve wasn't technically an illusion, it was Kayala's appearance in the giant poppy flower. Just gorgeous. I loved that whole scene and I'm still sad it's gone. The Kayala-painting illusion is good but doesn't compare to the original poppy scene, IMO.
I love Criss' entrance at the start of BeLIEve, levitating in the "Luxor light", and the whole "BeLIEve banner with Hammie then miraculous appearance in the audience" illusion
I always thought Metamorphosis was great, and the straight jacket escape -- it's even cooler now that it's done directly above the center of the audience.
Oh, and the ushers! (Are we still calling them that? Or are they just "clowns" now?) The trick that still makes me laugh every time is when Slim "levitates." Fufufu! Levi-ta-SHUN!