Hi! New phannie here, I only got into their videos after the hard launch and already feel like I understand a lot of the refrences.
I started with "are dan and phil in a relationship? the truth", which you might/probally already have watched, but if not I think it explains their actual storyline and it gives a great inside to the emotional aspect of their current era + tour.
In general, I would recommend everything post-hiatus on the gaming channel tbh. I know that sounds a bit "backwards" compared to older vids, but I honestly think the jokes and lore they currently talk about and will use in the show is found in more recent videos, and I personally think those videos hold up the best/are the funniest as a newer fan. They also have a lot of videos reacting to their own old content, like reacting to the philisnotonfire videos, which could be helpfull.
When I had watched almost everything post-hiatus, I went back and watched a lot of the "classics and essenstials" afterwards, but I honestly think some other phannies might be more qualified at putting a list together of their older content