I'll go with Stronger: Charge Up. Just for how simple it is, but the over the top one I like best is Decade.

Zolda Survive looked pretty good. If only it was cannon lol.

But of the main ones. King Form. Always King Form.

It's also great seeing everyone talk about how "merging with 13 Undead at the same time should be impossible."

Aw, no Showa power ups?
It's not just Stronger that has one! Rider 1 and Rider 2 both get upgrades, and Skyrider gets an ugly lime green suit (the one we see in all of the modern crossovers) and new powers after his training with the original Riders!

Also, no real spoilers... but are we really sure that PuToTyra is OOO's power up form? It always seemed to me like those were regular combos, and TaToBa was his "powered up" form. Being that it was the originally OOO's form and all.

indeed!!! I get a big ass stupid grin when I hear those chimes

Gotta love Kiva Emperor Form.

Man, OOO TaJaDoru hands down, that is my favorite kamen rider design to date

So if TaJaDor counts, why doesn't Blade Jack Form?

I haven't yet seen all of these forms (like Ryuki Survive and Den-O Climax, other than in toy form), and as much as I like PuToTyra/TaJaDor and Ultimate Kuuga, Emperor Form Kiva is definitely my favorite, and I expect it to remain so, as Kiva is still my favorite series.

I am actually wanting Cosmic States as a S.H.Figuarts. It's growing on me.