I can't speak for legs from most newer seasons, but there are some real good classics I can think of:
Season 9, Leg 6. This leg was great because of the self-driving, the beautiful countryside, the awesome thrilling Roadblock, and the entertaining Detour.
Season 6, Leg 2. Beautiful scenery, very entertaining Detour. I really liked the overnight rest with the boat race between the teams.
Season 11, Leg 8. This leg was actually really awesome. The visit to Auschwitz was wonderful and emotional. The Detour and Roadblock were both great, and made even more entertaining because this was a night leg.
Season 12, Leg 11. The finale leg had perhaps the most well-designed Roadblock in the history of the Race. Not to mention it was fun to see the teams ice climb, and skin fish.