Another vote for Turn A.
While it's not always an easy watch (slow pace, throwaway episodes...), it is is without a doubt in my mind the "best" Gundam series. Not my "favourite", but the one that encapsulates the best the idea of Gundam as a... I was going to say war drama, but I'm not sure it's the definition I look for when thinking of Gundam. It's part of it sure, but not just that. Damn, need to think more about that.
Turn A comes with an overall wonderful package to boot (visuals, music, battle choreographies, etc).
Love the setting, the cast, the war drama, the epic (which comes late, admittedly), even the mecha design (came to terms with it).
Maybe I should watch it again... It'll only be the 4th or 5th time.