Why not rot13 the alien monster dialog? The completist-watcher can then pause & decode the speech if they want. Come to think of it, that’s stupid. Now, with MKV, you can have 2 subtitle tracks, one with translated monster speech, and one without. Anyways, no more technical barriers. Now it’s a speech and translation barrier.
As for KR series, you will get completely different opinions on the worthwhile series, because each series is a like a mashup of a fixed set of themes, and each year they pick a few to keep, flip a few into its opposite theme, and throw the rest away. For example, I was just arguing with my friend today that all the new KR series has a lot of rider vs rider fighting, and he pointed out that I skipped all the series where this doesn’t happen (or not as often, anyways) because there aren’t many riders.
I think you will have no choice but to watch a few episodes of a series at the beginning, and if it doesn’t grab you, just move on to the next one.
Wikipedia may be able to help you with synopsis of each series, but there are lots of spoilers on those pages.
I can tell you what I’ve watched:
Kuuga: didn’t finish, looks really interesting, but I was watching Taiwanese DVD with excruitiatingly painful English translation. (Think babelfish, but written by Taiwanese.) Single loner rider who changes forms. Pretty, organic design. Most like the older riders.
555: didn’t finish, did not watch from beginning. No opinion about story and characters. I only have opinion on the rider costume design, which flipped from extreme hatred into stylish-in-an-amateur-sense…design is Greek alphabet based. One of the “belt is a Rider System” type of riders, where the belts can be worn by anyone.
Kabuto: Finish. Love love love, because the main hero is fights arrogantly and confidently. So refreshing looking. A lot of riders after this series fights like him, and henshins like him. “Rider System” type of riders. Lots of them, all doing things that sometimes conflicts with each other. I especially love ZaBee, Punch Hopper and Kick Hopper for their design and their silliness. Insect-based costumes.
Den-O. Not finish. Hated the characters, hated the plot, hated the rider design. Popular Japanese mythological characters-based.
Hibiki. Not even started. This is my friend’s most favourite Rider series. It seems to have done some very different things, like based on medieval Japanese feel, has a musical instruments theme, has maybe an ecology theme? Weird organic costume designs.
Kiva. watched 1/4 of the series. It’s actually pretty interesting, but I had to stop due to work, and never went back to catch up. Kiva is the only rider in this series (well, up to 1/4 of the series anyways), and changes form in such a weird way that you just know there must be a backstory. This series tells 2 stories in parallel, 20 years apart. Has vampires. Of course, Kiva’s costume is a Goth Kuuga.
Decade (current series): Totally watching. Silly homage to the previous 9 years of Riders. “villian of the week”, except it’s every 2 weeks. Decade has to travel to each older rider’s world and beat up someone/fix something. The world has the same riders, and the same monsters, but something doesn’t follow the original story. So, it’s way fun for people who watched that series, but it’s not too bad for those that missed it. I love the design of Decade, but hate DiEnd and Super Decade. Those are just disasters. Although Decade is just 2 riders, it has the last 10 years worth of riders around, so there are plenty of rider vs rider stuff.
Ryuki: never watched. I mention this because it seems an important series…the only one where there are no monsters for the riders to fight. Instead, there are 13 riders who fight each TO THE DEATH…THERE CAN ONLY BE ONE. My friend insists that this one is groundbreaking. Plate-mail-based costumes. Why in heck is THIS one getting officially translated for US?