For Longevity, nothing comes close to the St. Jude (standard) Mechanical Valve. It's been around for a LONG time and has been laboratory tested for more cycles than any of us are likely to see.
The new (third generation) mechanical valves (On-X, ATS, St. Jude Regent, Sorin, etc) have been designed taking fluid dynamics into consideration to reduce turbulence with a lower liklihood of producing clots.
There has been LOTS of interesting discussion on these new valves which you can find by clicking on the SEARCH tab at the top of this page and typing in the appropriate valve name as the keyword.
You can also find websites for the different valves by doing a normal (Google, etc) internet search or from links in the above cited discussions.
That should keep you busy reading for some time!