I think for now it is source, especially with the announcement that they will soon support dual (or more) cores.
For lognevity and number of games it would have to be the Quake 3 engine, many fine games were based on that.
The Cryengine should get an honourable mention, but as only one game has been based on it it is hard to judge on it's own.
When an enginge can be used to 'power' various different games then it has to be the best, so Q3 wins.