>>23489956
A starting point really depends on what you want in a series. My personal recommendations:
Build, Gaim, Zero-One and Geats all seem to be fairly common starter series for people since they're all-around solid and they have more serialized plot structure.
Kuuga, Ryuki, and Drive are series that are looked on favorably. Kuuga and Ryuki are way different so they don't set expectations but they are really good, Drive tends to get neglected because while it's a good show I think it's very safe and doesn't really have any major flaws but at the same time doesn't feel as ambitious.
Ex-Aid is quite good if you can get past the strange suit design. With the suits in mind being kind of off-putting I find it hard to recommend as an entry point, though.
The W-OOO-Fourze trio are still extremely strong series that showcase form changes really well and have a good balance of more serious and lighthearted elements. For most people the trio are really easy choices for starter shows.
Kamen Rider has a slightly older target demographic, although both are still shooting for elementary-aged boys.
Kamen Rider focuses more on a range of power sets and the form change gimmick. Suits tend to be bulkier and more complicated than Sentai suits. Kamen Rider typically has more serialized elements while Sentai is more Monster of the Week, though most Riders do still have a monster of the week format at least in the first quarter or up to the last ten or so episodes.
Gavv just ended and is a generally good season, and the upcoming Zeztz we don't know but it's likely that Toei will be very cautious with it since they want it to succeed to hopefully get Rider to gain international steam.