The mainline games are usually standalone aside from obvious sequels (Z1, Z2, alpha, alpha gaiden, etc.) and are mostly mecha from anime series. This is where all the fun crossover stuff is.
Original Generation I don't know a whole lot about, but I believe there's more of an overarching plot between some games, and the characters and mecha are all original Banpresto ones.
If you're just looking to play the normal SRW games, these are good starting points and have English translations:
>Alpha Gaiden
>V
>X
>T
Alpha Gaiden is a sequel, but most of the story is standalone and it's easy to find the .iso for it so you can try without paying. The others are all recent games that got SEAnigger English translations and you'll likely have to import them. These are also some of the only ones in English so I hope you know moon