Hmm... I'd recommend hitting up the pilot of G1. After that.. maybe look around for "The Best of" lists and watch those.
Oldie G1 ain't the easiest to watch. It was sex back in the day cuz it was just so.... frackin' larger than life! You hadn't seen much like it and couldn't wait to get your hands on the toys.
I don't care who you are tho.... after watching the G1 pilot and... hmm... what was that finale where in the end, the Autobots and Spike shot in to the air and blasted Cybertron back in to space? LOVED that episode!
Ah, "The Ultimate Doom!" Great saga!
But after you get the best of.. break open Transformers the Movie. That film has stood the test of time for me and then some.
Then the G1 season that followed it. I loved all those episodes even more than the original series. Especially "Call of the Primals", WebWorld, Grimlock's New Brain, Burden Hardest to Bear, and The Return of Optimus Prime.
After that are the Japanese series which I found hard as hell to get in to.
Robots in Disguise has what I felt was the best looking Optimus and while Armada started off as one of the worse things I have ever seen in my life, it had a HUGE and awesome finale saga that was worth sitting through.
I liked Super Link, but eventually fell out of that and REALLY enjoyed Galaxy Force, but still need to finish.
Beast Wars is Transformers at its very best, but you need a little G1 perspective to REALLY love it.
Happy hunting and make sure to let us know where you start an what you think!
Oh! And Transformers Animated.... started off kinda crappy, but really finished strong in the end.