Gekiranger and Den-O.
Because I wanna see an Urataros possessed Retsu :p

Stronger vs GoRanger
Kabuto vs Kyoryuger (Just for King and Tendou together)
Agito vs Kyoryuger
Gaim vs Kyoryuger
Magiranger vs Wizard
Black RX vs Zyuranger
Amazon vs Gekiranger
Super-1 vs Dairanger
Den-O vs ToQger
Hibiki vs Hurricanger
Blade vs Gekiranger

Some random ones with different connections

Akibaranger VS Kamen Rider G.

I'm not saying this will write itself; but both are parody series (Akiba being more mean spirited) so if this ever becomes a reality I think it could work out.

You found Akibaranger to be mean-spirited?

Ryuki and Go-Ongers. I need that US SHT void filled.

In reality I would love a full Den-O/Tokkyuger movie.

Good lord Hibiki vs Boukenger, because, why the fuck not :lolol.

I'd like to see Gogakiger vs Gaim, So many form changes it'll be full of funny craziness.

Gokaiger vs Wizard :lolol

That would be crazy :p

It would be crazy awesome, though I don't know if the universe could handle them both.