Time travel (backwards) is most likely impossible. There are some crazy ideas but I doubt that anything like that would actually work/exist. Breaking chains of causality, Grandfather Paradox, Many-worlds interpretation to name a few things. Stable worm holes, exotic matter with negative mass… nah.
That leaves time travel to the future. Future prospects are… well… not so good in my opinion. Since nobody knows what will happen in the future:
How would anyone know when to stop the travel to the future? How would one know if they just left a good time period behind (and going backwards isn't possible)? Classic literature, like H.G. Well "The Time Machine" simply assumes back- and forth travel, so the time traveler can have a look at various future time periods while always having a way back. The future is not a good destination. Maybe the traveler stops right after a nuclear war. Then they have to go even more to the future hoping things will improve again.
I hope I remember that correctly: Futurama (although allowing backwards time travel in some episodes) has one episode where the professor invented a time machine that only goes forward. They solve the problem by just having a cyclic universe and fast-forward to the next Big Bang (multiple times). Chances are this isn't possible as well as the universe will, according to the current state of knowledge, either experience the heat death (Big Freeze) or even a Big Rip. →
[Wikipedia] Ultimate fate of the universe although there are other ideas as well.
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Leaving all the problems about backwards time travel behind and simply allowing a magical time machine that goes wherever I want:
probably the 1980s or 1990s. Freedom, education, less prudery, less technical possibilities for "Nineteen Eigthy-Four"-like total surveillance, overall less paranoia. Not having the risk of getting tared and feathered for deviating opinions or for making a joke poking a holes in each others peculiarities. All in all the most desirable time period for me (in western civilization).
Going back further would certainly not be an improvement over the current situation. The World Wars, the time before/after the wars… no, thanks. Not even talking about pre-industrial society – that life surely was hard.