Quick correction on the lists:
Decade: Can only turn into the previous Heisei Riders, Complete Form can only bring out the Rider for the hissatsu/super attack (I don't recall the Rider called ever doing more).
OOO: ALL forms have access to the MedaGabryu.

Actually, it's not.
Double's Final Form Ride is to split him into CycloneCyclone and JokerJoker and OOO doesn't have one (To my knowledge).
There is no logical reason for Decade to use it in this battle.

OOO, on the other hand, gains a slew of new powers determined by the combo that aren't available to the individual medals (ShaUTa's water transformation, SaGoZoh's gravity control, TaJaDor's flight, etc), making it much more relevant.

Eh sorry but no one can ever convince me that Decade isn't the most powerful rider. Just based on how his powers work and his (theoretical) ability to turn into anyone (as shown more of in Super Hero taisen).

The only time I recall the other riders appear is because Decade didn''t summon them, DiEnd did. And if Decade summoned them, it was only in their most powerful form via KTouch. The reason I said that the ability is irrelevant is because this is a battle between these 3 and these 3 alone, no outside riders are in the battle, and therefore, Decade can't use his FFR cards

We haven't seen him able to turn into the riders that came after him, the only "New" transformation he had was Ichigo.

But he's not the most powerful. I still firmly believe his fights in [Forgot the name of the movie] were pretty much set up so he could win (Or his opponents held the idiot ball so tightly so that he couldn't lose *Cough*KABUTO*Coughcough*)

Also, Decade can't summon Riders. That's Diend's ability. Decade can only transform into them. So unless there's interference, the only Final Form Ride available to Decade would be the Double CycloneCyclone/JokerJoker.

As it stands right now, I think that Decade has the best chance of beating OOO, taking everything into account, and even then OOO would probably still curb stomp Decade.
I ask: What possible advantage does Decade have that OOO can't immediately counter?
(Double doesn't even factor. He's awesome and all, and if it were a popularity contest, he'd reign supreme, but when it comes to power, he's a couple tiers too low to contend)

Of the three listed he is definitely the most powerful. Between the Kamen Ride and Form Ride cards alone he's a serious powerhouse. Complete Form being able to summon other ultimate forms, though, makes him easily stronger than Double or OOO.

That said, I will always maintain that Kuuga is the strongest Heisei Rider. Probably the strongest Rider ever.

In complete form he can summon the final form versions of the previous 9 heisei riders.

The way you said it you made it sound as if he could freely summon other Riders a la Diend.

Anyway, I guess I'm letting my issue with Decade's lack of character cloud my judgment lol.
Still standing firm with OOO though.

I don't think he ever used it as TaJaDoru (correct me if I'm wrong though)