People will tell you to either start with the original show or Kuuga to get a feel for Rider, but I'll say pick one that looks the most fun to you personally (because you like the costume design or the premise) and just watch it.

Don't let others decide, because while most people like a certain show some people hate it so they will argue about what you should watch because they like the show better and it sucks.

Just pick what you want and roll with it. There are some bad shows, but you won’t end up hating the whole franchise if you start with a bad one. My second show was rather meh but I kept going anyways.

Personally, I think there are a few choices. Kamen Rider Fourze is a very upbeat series with fun characters, and a lot of heart. Kamen Rider W is a Noir style almost, but it's story is really easy to get into. Kamen Rider OOO is another good choice for those reasons. Kamen Rider Kiva is an outside choice. While it has good characters, and some of the best music in KR, It has a few plot problems that might be distracting. But the soundtrack is worth checking out if nothing else. More recently there's Kamen Rider Gaim. It's one of the more original KR series', and a lot different then what came before.

Truth is there's a lot of good options on where to start. So it all depends on what you were looking for.

Ghost just began and we're 2 episodes in, so that would be a great place to start at. But if you want to watch a series all at once, all the Neo Heisei (Post Decade) series are all great.

Roll a 20 sided die.
If you hit Kuuga or Faiz, reroll.

But really, just look at pictures of all our Riders, find one that looks cool, and roll with that. If you're stuck, just watch Double or OOO. Both are great series to watch with very few detractors (While Gaim is also fine, it strides in changing up the formula).

It depends on what you like. OOO has a shonen anime feel to it, so that's not a bad place to start. W has a good mix of drama and action, so W is also good too. Gaim is great place to start as well. Kuuga is good, but the show has not aged well with its visuals. The CG looks like a N64 game now, so the CG may turn people off.

A quick word about Kuuga is that it very much moves by the beat of it's own drum.
While it does give it an air of originality, it also ensures that there is no sense of pacing.

I think Gaim would be kind of a poor place to start, personally. Part of what makes it so interesting is how different it is from the rest of the Kamen Rider series, something that would be completely lost on anyone who hasn't actually seen any other Kamen Rider.

Gaim has a really strong story to it which is why I feel like it's a good place to start. It still has the basic Kamen Rider stuff in it like able to transform with the belts, monster of the week and they ride bikes to travel.

Which super sentai actor would you like to see play a role in a kamen rider episode or movie?

Gaim is my favorite Rider series, and I don't know that I'd want to start with it. I wouldn't want to judge other Rider shows by its standard. Some I'd consider instead:

W - Strong story, strong characters, cool theme, and the toys don't overpower the rest of the show. Ends strongly for a Kamen Rider. Balances humor and seriousness really well.

Den-O - One of the funniest and most emotional shows. There are a few good reasons this show was insanely popular. Even a problematic mid-series casting change was handled... well, better than I'd have expected.

Fourze - The high school theme and core cast worked really well. Utilized its theme - friendship - remarkably well.

I haven't finished OOO, but so far, I can see why people are recommending it. Similar to W in that it doesn't feel to me like there are many filler episodes.

Let us know what you end up picking, Neptune!