TNG is where most people start, but I’d actually recommend watching the TOS movies first, personally. You can skip the first and fifth movie, the rest are pretty good.
I really enjoy TOS, but I know that a lot of people don’t have tolerance for 1960s productions. The TOS movies, starting with the second one, were made with a more modern style. You should watch them if only to get the proper impact from TNG’s ‘Relics’ episode.