That is hard to answer because I enjoy my current moment in life just as much as I have enjoyed the past. Also I do not want to live in regret or think of regrets.
Outside of taking a philosophical standpoint, I will answer your question. I wish that I can be 16 again. It was the time that I felt like an independent adult. I was going to high school and I had a part-time job, but I didn't have any responsibilities outside of those. I was an adult kid. That is a good cheat. Old enough to do things, but young enough to blame adults and parents.
Probably the 1910s)) because I would like to see for myself Saint Petersburg when Russia was still under Monarchy. Royalty is rare nowadays and I like history so the early 20th century would be interesting for me