Except for the first four seasons, all the subsequent Gundam season are set in separate universes. You can start with either of them really. For a more “modern” season, I would say the best is Gundam 00, if you are willing to go a bit further back, there is Gundam SEED and Gundam Wing. SEED is the only season beyond the original 4 to get a direct sequel so that tells you how good it is. Beyond that, there are movies set in the same universe as existing seasons, depending on their popularity.