I think GenArts Sapphire has the best 3rd party transitions (and other FX), but is also among the most expensive plugin sets. Boris Continuum Complete is also good, but also expensive. The Cycore ones bundled with AE Pro are good too. Pixelan SpiceMaster is an extensive and customizable set of various types of wipes, which I use a fair amount…like most plugins, it really only makes transitions that can be done with AE alone faster and easier.
The ones above are mostly of the more subtle type (wipes, blurs, etc.), which I prefer. If you like/need more flamboyant types of transitions (fly-ins, DVEs, etc.), you might look for an old copy of Pinnacle’s Hollywood FX…I have (and very rarely use) one from years ago that still works with AE6.5, but I’ve heard thier newer plugins aren’t AE compatible.
Hope that helps.