>>717731558
Gaim is the closest a main series will get, but the standalone movie spinoff The First is probably your best bet. The First is pretty good and you can go into it blind but it's not very representative of what you'll get out of the series in tone otherwise. And whatever you do, don't watch The Next, it's a dumpster fire.
Also don't watch the recent Shin Kamen Rider either, it's a movie specifically made for the fans, you won't appreciate it if you aren't already familiar with the series, never mind its own issues.