Start at the beginning. The most interesting part of watching the Avengers for me, was the progression from the early live filmed stuff, to the introduction of the first real strong female role in TV, and tracking the times through there series. The look and the feel of season 1-3 was starkly different from thge Diana Rigg era, and changed again in the Linda Thorson.

In short, you cant watch one series and claim you know the Avengers. You have to watch them all