If you're looking to get into the main timeline of Gundam (Universal Century) I'd probably recommend starting with the
3 compilation films of the original 0079 TV series or the 0080
War in the Pocket OVA or the
08th MS Team OVA before moving onto the
Zeta TV series.
If you're looking to sample one of the AU Gundam shows that's pretty self contained and easy to watch I'd probably recommend
Gundam Wing (it's my favourite) or
G Gundam (which is hilarious).
SEED is okay (the characters are a bit obnoxious), but
SEED Destiny is an absolute trainwreck of a show that I really couldn't recommend to anyone.
Gundam 00 is also pretty fun for the most part, but again the second season was a bit of a disaster.
Of the more recent Gundam series I have to say that
Gundam Build Fighters is fecking amazing, but having a few Gundam shows under your belt first massively ups the enjoyment factor. It is a show that is made by fans for fans of the franchise and is full of homages and Easter eggs.
Gundam AGE was very decent, until it took a nosedive in the 3rd act and ended on a bad note.
Avoid
Gundam G no Reconguista like the plague - worst show I have seen in a while, it made absolutely no sense at all! May also want to skip
Turn A Gundam until you've seen some Universal Century shows - ditto for
Gundam Unicorn - both are excellent and mostly stand alone, but do make more sense if you have some understanding of Universal Century events.
Hope that helps a bit! I do enjoy Gundam