What platform? Do you care about multiplayer? Demon's Souls and Dark Souls 1 probably have dead player bases on consoles. All PC games are still active.
You could start with any of them just fine. However, Dark Souls 3 might be too fast paced for beginners, while DS2:SotFS often forces the player to take on too many enemies at once. DS2 has some really odd design choices that make the early-game annoying to play through (e.g. need to level up Agility to get a decent roll). DS1 has slow combat that's easier to learn, plus you can use a bow for luring out individual enemies, which rarely works in DS2. Demon's Souls is great as well, but has some outdated mechanics (e.g. there's a limit on how many items you can carry so you have to store them at the hub). I can't comment on Bloodborne as I haven't played it. Overall, I think DS1 is the best introduction to the series.