Might make a list of starting shows later on
So here we go, as threatened. This is in no way definitive, based only on stuff I've seen and only complete shows, meaning none of the current iterations of the "big 3" (Mashin Sentai Kiramager, Kamen Rider Saber and Ultraman Z). I'm also trying to stick to around 3 for each. Might do some "Miscellaneous" later on.
SUPER SENTAI:Choujin Sentai Jetman (1991) - The last of what I would deem the "Military Sci-fi" era of Sentai, or of the type that follow the blueprint set since 1975. An invasion by aliens or in this case, beings from another dimension, with a team of heroes supposedly trained for such an eventuality. This one has more "drama" in it than Sentai was used to (love triangles and stuff), but also just nails what makes a good sentai team. Suits INCREDIBLY derivative of Gatchaman/Battle of the Planets. Also pre-Zyuranger, meaning none of this was adapted into Power Rangers. Written by the infamous Toshiki Inoue (when he was actually good) and Directed by Keita Amemiya (more on him later)
Gosei Sentai Dairanger (1993) - This one is about the long-running battle between two tribes that continues into the present day (i.e. 1990s Japan). Draws heavily on Chinese mythology and Hong Kong movies (some cool martial arts fights in this one). Notable for being used to make season 2 of Power Rangers, although only the monsters, mecha and Kibaranger were used (the latter being the 6th ranger of the team, used to form the basis of Tommy's White Ranger powers). It's criminal how little of this got used. Gets dark in its later episodes, without giving too much away.
Kaizoku Sentai Gokaiger (2011) - The 35th Anniversary I debated whether to put this forward, but I think it works regardless of how many prior Sentai you've seen. Five space pirates come to earth in search of treasure, end up having to fight off an invasion by an evil space empire and learning what it means to be a Super Sentai team in the process. They also have access to the suits and powers of the previous 34 teams. Cameos a plenty but a lot of the time the show will establish who certain people are pretty quick. The 6th Ranger is a human Sentai fanboy who kinda explains to the Gokaigers who these people are. The 2 teamup movies are also great.
199 Hero Great Battle is worth it for the opening 6 minutes of the "Legendary War" and
Gokaiger vs Gavan is a great comeback for the legendary Metal Hero, Uchuu Keiji Gavan (Kenji Ohba is awesome).
KAMEN RIDER:Kamen Rider Black (1987) - Picked this because of all the Showa-era shows (Kamen Rider is commonly split up into eras based on Japanese Imperial reigns), it seems to be one of the most standalone series(Until you get to its sequel, Black RX, but that's another matter). Classic stuff, young man turned into cyborg bugman by evil cultish organisation, escapes before they can brainwash him, uses his cyborgness to fight said organisation. Much angst about the loss of humanity and family in this one.
Kamen Rider Agito (2001) - The second show of the Heisei-era. If 2000's Kuuga was Christopher Eccleston, this is David Tennant in a way (partly because like Eccleston, Joe Odagiri won't come back for anniversaries and movies). This really feels like where the Heisei era tropes start to form. Multiple Riders, Toshiki Inoue bringing his soap-y stylings to the table, less cyborg-y stuff, different kind of angst.
Kamen Rider Gaim (2013) - Do not let the fruit elements of the suit designs, or the dance elements of the first quarter put you off. This quickly gets serious after that and the stakes get stupidly high. If you know anime, you likely know the writer for this one, Gen Urobuchi, best known prior to this for Madoka Magika. As I say, high stakes and consequences that are not shied away from. There's an anonymous blog post people take the piss out of on Twitter relentlessly where a Gaim fan talks about this Toy Commercial as if it could air on HBO alongside
Game of Thrones. That's silly, but there are times where you have multiple factions, shifting alliegances and twists that it's easy to see where that view comes from.
ULTRAMAN:Ultra Q (1966) - Very much "early installment weirdness" in TV Tropes speak (sorry), as this IS the first thing in the franchise, except Ultraman isn't in it, yet.There are kaiju (some of which get reused up to the present day), but the presentation here is more a Twilight Zone/Outer Limits kinda show as opposed to the superhero stuff introduced in the original Ultraman show. If you're familiar with Toho movies, you may spot reused monster suits (like the modified Godzilla suit in episode 1 and the King Kong suit later one) and props.Tsuburaya would return to this idea later on with Neo Ultra Q.
Ultraman (1966) - Ultraman is relatively pure in terms of the show format and any gimmickry. This has the formula established from the off - a government agency set up to investigate weird scientific phenomena and protect the world (usually just Japan admittedly) from monster attacks. An Ultraman bonds with a human host or assumes human form to aid said agency, sometimes as a member of this group. That's true of the current show as well.
Ultraman Geed (2017) - This is my personal favourite of the "new Generation" Ultraman shows, also a bit heavy on continuity, in that Geed is the son of Ultraman Belial, an evil Ultraman, and Geed is somewhat mentored by Ultraman Zero, himself a son of an Ultraman (Ultraseven) and the one who usually fights Belial. The powers used by Geed are also representative of Tsuburaya's current tendency to make "fusions" of older Ultras. You can still get into this easily enough without knowing the Zero/Belial backstory. Likeable enough cast here.
This one is also available to stream via Crunchyroll, free if you don't mind the ads. Or get the Bluray set if you want physical media.