The Seagate Momentus XT hybrid is the quickest you can get by a large amount. Well worth the spend over a regular disk if you can't go SSD+HDD.
It's only the quickest drive IF the data you want to access is stored on the 4GB of solid state memory, if it's stored on the mechanical platters then it's not the fastest at all. The Western Digital Scorpio Black 500GB still holds the top spot for fastest 2.5" HDD (not counting enterprise 2.5" drives)
StorageReview have a good review of the Scorpio Black here: http://www.storagereview.com/western_digital_scorpio_black_500gb_review_wd5000bekt - They also have a nice section on the current, fastest hard drives around here: http://www.storagereview.com/best_drives
Hybrid Hard Drives - Seagate is the first to re-enter the hybrid hard drive space with the Momentus XT. The drive comes in capacities up to 500GB and beats out all other notebook drives (the VeoliRaptor too for that matter) in our real world tests, where the drive is able to effectively use it's extra 4GB flash storage. The 500GB model runs roughly $130, an excellent value for top-line performance.
If it's for HTPC use then you might well see an advantage, for most other uses I've seen very little gains (from personal experience). I found windows would boot up quicker and Chrome would open faster, that's it - maybe if they increased the flash to say 10GB it would be more useful, I personally think 4GB is too small to be any real use considering the size of most applications these days.