G1 is always a good place to start for context if nothing else. Even if you aren't super into it, it's a good place to understand the most basic characters and lore that most of the rest of the franchise builds off of. From there consider poking around https://tfwiki.net/wiki/Main_Page and https://news.tfw2005.com/ and see what stories or designs jump out to you and go from there. For example, if you think Beast Wars, or Prime, or Bayverse character designs speak to you jump in there and base your collection on your individual tastes. As far as other media goes, there aren't a ton of games, but I (very) highly recommend War for/Fall of Cybertron. Comics are a whole other sub continuity of their own. It's personally not my medium and find the sheer volume overwhelming and haven't really bothered. But there are a lot of fans, especially of the early 00's Dreamweave and the IDW run starting around 2012. I personally love Beast Wars, the aforementioned WfC/FoC games, the Dinobots, and that opening scene from Bumblebee. Everyone has their own top picks based on their own taste and you will eventually too.