I told myself I was just in it for the cool toys and not any of the fiction, but if I wanted to at least get some familiarity with the franchise outside of the toys, where would I start? I've seen/read none of it outside of maybe some BW episodes whenever those aired.
In terms of cartoons, G1 is your basic core. The complete series is
$50 on Amazon (Transformers The Movie not included, and strangely hard to buy legally), assuming you can handle 80's toy-based cartoons or have a nostalgia itch to scratch.
Beast Wars (
$20 on Amazon) is a good spinoff/sequel to G1, assuming you can handle 90's CGI. Don't bother with Beast Machines (the sequel to Beast Wars) as it's kind of crap.
Transformers Animated (
$25 on Amazon) is a somewhat more lighthearted and fun cartoon with lots of G1 fanservice (stuff like having Weird Al guest star as Wreck Gar).
Transformers Prime (
three seasons and a
movie) is the most mature take on the cartoon, and it's probably the best Transformers series.
There are other series, but these are the big ones. If you want to shorten the list, the most important one (culturally speaking) would be the G1 original. Everything springs from G1.
The Michael Bay movies are crimes against humanity. Sadly, they might be more culturally relevant than the rest of Transformers combined. Please avoid them.