Kamen Rider Vs Super Sentai.

All Riders Vs All Senshi.

That turned out so well last time :lol

Oh-Ranger vs Shin!

We got that movie.
It doubled as Decade vs Gokaiger, too.

I was thinking more "199 Vs 97: SUPER HERO TAISEN GIGA:" a big 199 hero-style battle between EVERYONE.

Yuusuke with King and the others would be absolutely hilarious.

Ironic part about Faiz vs Go-Onger is that the Red Falcon Mecha is voiced by the guy who plays Faiz.

I think your post might be a bit misleading. Engine Retsu-Taka is voiced and played by Faiz' actor (Not to be confused with Engine Speedor, Red's Partner). The Falcon is the one they use in Gokaiger. Speedor is a Condor and Retsu-Taka is a Hawk.
Side note: The other two movie engines were played by Goggle-/Dynablack (/Souji's father in Kyoryuger) and DekaPink.

Den-O - Gekiranger. Den-O was crazy for the most part, and from what Ive heard, so was Geki, to see those two together would be awesome :lol

And Ryotaros Imagins all correspond to one of the Gekiranger :lol

Could go mad with this!
 

I picked W and Goseiger as I liked them both (especially W). Frankly one could go mad with this. Some other team-ups could incorporate:

- Aoranger (Goranger), Big One (JAKQ), Kamen Rider V3, with cameo by Zubat.
(Same actor for the human side of all four)

- Kamen Rider J and Zyuranger (specifically TyrannoRanger)
(Same actor again)

- Kamen Rider Accel and Go-Busters

I know others have already touched on some good ones.