I'm not so sure why this movie is getting a "4/10" on IMDb. Honestly, I don't think this was a bad movie at all. Now, it's not nearly as good as Movie Taisen Megamax, in my opinion. But, I will say that it's definitely better than any of the Super Hero Taisen movies, except for Z. Now, I wasn't a fan of the showa-era Kamen Rider shows, since I never really did grew up watching them. However, I will say that this movie actually made me appreciate the established history that it created from the original Kamen Rider series to Black RX and I might actually check out the showa-era Kamen Rider shows, someday. And, unlike Kamen Rider Taisen, Takeshi Hongo (Kamen Rider #1) is not an asshole, he's actually supportive and he fully acknowledges the current Kamen Riders; Ghost and Spectre as real Kamen Riders of the Heisei era. The villains of this movie actually not bad and I really wanted to know more about the Nova Shocker group and how they deflected from the original Shocker group.
Overall, I think Kamen Rider #1 (The Movie 2016) is really underrated and I would only recommend this for any one who is an old-school fan of the showa-era or if you're like a new-comer that really wants to see an old-school hero coming out of retirement and doing what he does best, then yeah. This movie is definitely for you. Or you could check out Kamen Rider Blade: The Movie - "The Missing Ace" as an alternative.
Final score....8 out of 10.