IS is really just a harem anime, not sure how i got wrangled into that for as long as i did.
I mean you can easily start with almost any Gundam and get a relatively similar experience, personally you could watch Gundam Unicorn and save yourself the trouble in going back to 80s/90s animation, all the while getting the gist of what the hell Gundam was shooting for all these years plot/thematically. Also it has one of the best mobile suits in the form of the NZ-666 Kshatriya.
Forgive my crappy non-player link.
https://www.youtube.com/watch?v=nKXSn1ZxrLE
Anyway, their are so many mech animes out there(albeit most of the follow the same tried and true formula).
I mean to second a few of the earlier comments:
Code Geass - I guess it's death note/mecha, i mean anti-hero protagonist are really the only similarities and such, good show, oddly doesn't age too well. But....
Code Geass: Akito the Exiled - Newer OVA, yeah yeah requires code geass to "get", but still a generally more interesting protagonist. I actually regret that prior sentence...fuck it, committing to it. No one call me out.
Eureka Seven/Astral Ocean - Solid show, differs itself from most other shows due to its interesting premise, good characters and such. AO is slightly less impressive, but mainly because the MC isn't as interesting as his parents.
FLCL - I'd personally say a must watch. 6 episodes anyway. Nothing else quite like it, not really a mecha anime also. The feels though.
Big 0 - I want to call it a classic, i fucking really do, but main that thing needed a second season. I'd reccomend watching it for the old school episode structures/scenarios, but good characters, good mech design and a setting that needed to be expanded upon.
Samurai 7 - Like FLCL, its got mechs in it, not really a mecha anime. Excellent show on its own however.
Zoids? - Man a zoids nod. Not even sure what to say. It's certainly unique, pretty fun but forgettable show. I'd say watch the 1st series and then lay it to rest.
Macross - Also a classic, if you're already gonna time travel with Gundam, might as well make that shot a double.
Shows no one hopefully mentioned so far--------------------------
Broken Blade - Yeah its a pretty straightforward MC finds mech, things happen, but the setting is certainly interesting. The show has its moments for sure.
Aldnoah.Zero - im sure between me typing this and posting it, someone will bring it up. I'd say fucking watch it, its certainly an excellent addition to the genre that champions tactics to win and a interesting(if kinda standard) plot.
Buddy Complex - Im pretty sure i enjoyed the show, yet couldn't remember its name for like 3 mins over here sipping whiskey. Besides the name and time travel nonsense, its a pretty fun ride.
That one anime i forgot which the like 18 kids gotta pilot the mech using their life force to save the planet and no one gives a shit about their sacrifice show???? cant remember that 1's title either.
Oh well, probly missed a few.