Personally, I'd start her off on Prime or Animated and work my way down to the older stuff. The fact of the matter is that most people who didn't grow up with it really won't have much interest in watching an old 80s cartoon. Hell, I grew up with G1 and still can't imagine ever bringing myself to watching it again.
I'd prefer to end on a good note with Prime. Plus, by showing her some G1, she can relate better to what they have in future shows for storylines and characters that are inspired by the original, rather than me trying to explain it to her.
We watched the '86 movie last night, and I wanted to let you guys know that my wife ended up liking it pretty well. She thought it was awesome when the updated hair band Transformers theme came on during the opening credits, and couldn't believe it when Optimus died, as well as Starscream (if anything, she had the biggest reaction to Starscream). Overall, she agreed that the animation and writing was much better than the regular show, and she liked getting to know the characters pretty well compared to only recognizing a few TFs here and there during the episodes we watched.
HAHAHA that is ALSO my favorite Skywarp line, and the exact thing I think of about that episode!! Anyway, here's a good list for new folks: More Than Meets The Eye 1-3 Fire on the Mountain War of the Dinobots The Ultimate Doom 1-3 Enter the Nightbird Attack of the Autobots The Master Builder Megatron's Master Plan 1-2 A Decepticon Raider in King Arthur's Court Golden Lagoon The Gambler Triple Takeover The Key to Vector Sigma 1-2 War Dawn Cosmic Rust Starscream's Brigade The Movie Five Faces of Darkness 1-5 The Killing Jar Starscream's Ghost Grimlock's New Brain The Burden Hardest to Bear The Return of Optimus Prime 1-2 A long list, but they all seem essential to me of the original series.
Other than the usual recommendations such as MTMTE, I would recommend War Dawn also because it shows the origin of Optimus Prime as it shows how "normal" things were before the war escalated. I remember watching that as a kid and was pretty impressed - pretty good back drop story for its time.
