I think the best opening leg was TAR 6 in Iceland. I'll list reasons why I think it is the best.
1. It was two hours. The two hours means you could fit eleven teams into one episode and still make every team have a time to shine. Even the first team out was memorable in a way.
2. We got to know the teams quite well. If you want to make a good opening leg then at least let us know why the team wants the million, what their personalities are, why they want to run the race, etc.
3. The self driving. I think what makes a good opening leg is to have the teams drive themselves to locations. It can be a bit boring at times, and drags too long, but if you have fighting couples and teams who don't know where they are going, it makes it fun to watch.
4. It set the couples mood of the season. There were a lot of couples in this season. By the way this opening leg was edited you could clearly tell that this is going to be a couples season. The final six alone were couples that are either dating or married. I would just call TAR 6 the couples season (I know TAR 26 is the dating/blind dating season), What a coincidence twenty seasons later there would be another one.
5. The leg design. Why have I been talking about the editing so much, let me talk about how great this leg was designed. It had teams start by the Buckingham Fountain in Chicago. Teams had to take the famous "L" train in Chicago to the airport. Three flights are available. Teams had to drive themselves to the first destination. Then have to drive even more to a perfectly place overnight rest destination. Drive to a Detour. The Detour although weak is a classic deciding to climb an ice glacier vs finding a buoy in an iceberg field is amazing. Then teams have to self drive themselves to the Pit Stop.