Scott (apparently no relation to Scott Von Schilling) of Mechanical posted a little mecha top-5: FMP, Geass, Gurren-Lagann, Escaflowne, Gundam 00/Gundam: IBO. Each generation has its own heroes, I suppose. Still, I think we can do better without going full Let's Anime.
Gurren-Lagann: This is one entry that Scott selected appropriately. It is a masterpiece. That said, my relationship with it changed with years. Before, I thought the post-timeskip was too edgy, with the trumped-up chargers and death row, people die when they are killed, and all that. But now, it's all I rewatch. The pre-timeskip story seems a little too... shounen? For the lack of a better moniker.
SDF Macross: The only Macross worth your time, even with how long it is, and how much of a stupid broad Minmay is.
Martian Successor Nadesico: I never rewatch it, but I thought it was surprisingly good.
RahXephon: Better than EVA.
Gundam 08th MS Team: If you only watch one Gundam, make it this one — not the kids' stuff like 00 and Wing.
Now that was a real Top-5! In addition, here are runners up with honorable mentions:
Gundam Build Fighters: If you only watch two Gundams, make sure this one is the second one. It's the Gundamest of all Gundams, a thrill end-to-end.
Dai-Guard: I adore this, but I admit: it's not on the same shelf as TTGL and RaX.
Patlabor OAV: Way to ruin the series by giving it no ending.
Vandread: The only good series by GONZO. It had its moments. My favourite is Barnett.
Stellvia: Tatsuo Sato was still pretty good in it, although not as good as in Shingu and MSN.
Kights of Sidonia: It is 3D, but... not too bad.
Darling in the FranXX: I haven't completed it yet.