When I had it, I would say the 85 1.8. I feel it often gets overlooked by CR folks (and folks on other serious photo forums) for the far more expensive, heavier, slower 1.2 version. While the 1.2 version produces some amazing results (and I may end up with one eventually) the 1.8 offers one of the best price/performance ratios in the Canon system IMO. Compact, inconspicuous and has decent reach and speed for close-range sports on 1.6x (bonus points here - the 1.2 is too slow for sports on a non-pro body). Really doesn't get much better than that...that and the "sister lens" 100 f2.
Only reason I gave up the 85 was to help fund an upgrade from the 70-200 f4 to the 70-200 2.8 (don't regret at all...would do again)...still though, that was a tough lens to part with. There has been many a time where I either wished I had it or contemplated re-purchasing it...probably would go for the 100 f2 if I did it all over again.