I only properly played 1, 2 and a bit of Black Flag but based on that and what I’ve seen from all the other games I’m gonna stick with the first one.

Investigations were… well actual investigation, gameplay mechanics while simple and satisfying weren’t overly automated and the game wasn’t burdened with all the bloat that came afterwards. Simplified movement system from later games, one that’s fighting you whenever you try to do something even a little out of game’s comfort zone, is probably my major sticking point with the series.

That said, I’m not sure if that would be the best choice for you. If you want to try the classic approach I’d suggest going with the Ezio trilogy (II, Brotherhood, Revelations) as these games are more polished, if a little bloated, compared to the first game. They should still hold up well enough to have fun.