Glass SEALANTS and glass cleaners are not to be confused.
AG Glass Polish is a heavy chemical cleaner for glass, not a sealant.
I used Dodo Juice Supernatural, Gtechniq G1 and Carpro FlyBy30.
I prefer the Flyby30 personally but that's just because I can get it cheaper than everything else.
The Dodo Juice Supernatural has the nicest bottle and is lovely and easy to apply.
The Gtechniq G1 needs the residue remover afterwards which is a bit of a pain however it does last the longest, a full year.
Ultimately most of the higher end sealants are about as good as each other.
Rain X is not what I would call a glass sealant as it's really poor for durability. It's just popular because people can buy it in Halfords.