While I can't subject myself to just lay claim on any one "great" jump, I'll name a few.
Scott Hall & Kevin Nash to W.C.W.: The beginning of the ratings war came with Scott Hall entering through the crowd & proclaiming a war. It thrived when Kevin Nash joined, & it'll forever be remembered as one of, if not possibly the, best jump of all time.
Kurt Angle to T.N.A.: Basically the latest shock-wave sent through the wrestling world. Rumors had circulated (sp?) that Angle wasn't at 100% & his health was a huge issue, thus the W.W.E. allowed Angle to get outta his contract. A mere month or so later, he shocks the world by joining T.N.A. & driving a huge blow into W.W.E.'s gut.
E.C.W. Wrestlers to W.W.E.: I understand E.C.W. went under & it was foreseeable that some came to W.W.E., but when Paul Heyman suddenly replaced Jerry Lawler, then shortly after Rhyno & Spike Dudley showed up on Raw in the same night. It was unbelieveably amazing. Same goes for R.V.D. & Tommy Dreamer with the Invasion angle, sure it may of been a matter of time.. but noone knew when.
Radical Movement: By far, the best group jump has to go to Chris Benoit, Dean Malenko, Perry Saturn & Eddie Guerrero. As its been said before, this was the nail in the coffin that killed W.C.W. To lose any ONE of these guys is huge, but to have them leave together, then debut together was deadly.
Raw is Jericho: I didn't live in the internet world of rumors then, so this was a huge shock to me. Jericho wasn't exactly Main Event material in W.C.W., but was definately a huge talent to lose.
Bret "Hitman" Hart: Again, sure it was foreseeable to some. But to those without the internet to fall on, noone knew Bret was leaving the company. So for him to debut on Nitro, it was huge!