Depends on what you're into. G Gundam and Turn A Gundam what I'd recommend to someone who doesn't really like the "standard Gundam formula," which you will see for yourself after watching multiple series. I started with the original series, Mobile Suit Gundam, and watched the Universal Century timeline chronologically from there (the OVAs in between->Zeta->ZZ->Char's Counterattack->Unicorn->F91->Crossbone manga series->Victory). My personal favorites are Gundam/Zeta Gundam (I consider them one experience), Turn A Gundam, then a tossup between Wing, G, Victory, and X.
Stay away from SEED.