Resident Evil series, never played

I have a few questions about the Resident Evil series. I’ve played Resident Evil 6 once, but I’ve never completed any of the games. Since the first trailer for Resident Evil 9 was revealed, I’ve been interested in playing it. Before that, I’d like to get more familiar with the series.

I’ve decided to complete Resident Evil 2, 4, and 6 before playing RE9. Which games do you think I should play before RE9 to deep into the story?

Edit: I do grateful for your advices and appreciate it. So, acc to your commets, decided to play first 2,4,7 and 8. If i get more time, I will deep into other series.

Edit2:
Thanks so much.
I add RE1 into this list too)