I actually recommend 12, 17 and 25 if you focus on task. GOLD seasons like 3,5,6 are great, but the task is rather simple to somewhat dull compare to the recent seasons. (I mean come on, if running around fountain is used as RB nowadays we would be bitching about it forever.) But the authentic feeling of location and actual race, navigation with a lot of flight drama and self/driving and also the editing and teams' interaction really made up for this and make it super entertaining.
So if you watch the older season, you should keep in mind that the task and leg structure might not the the best since it's still trying to find its own identity and kinda in the transition state. (It become more stable in season 12 and onward.)
Oh, and check out TARAus2. It's AMAZING. <3
PS. I also think 18 is actually an amazing season. But I didn't suggest because it's a returnee season. So I think you should maybe watch it after 12-17 for the full entertainment. <33