ok, the Diend bailing thing...yeah yeah yeah...but this is a fight...

I still say Diend will win. He'd cloak, have Agito wear Caucasus down, then blind side him with his own troopers, when Caucaus is finally worn out, Diend would appear behind him and final attack ride him...

I'm voting Diend here. While it's true he's more of a thief than a fighter, I think if he had a solid reason to fight, he'd win.

To not go too insane, I just want to point out that slowing time to the point that it's perceived as stopped to a being in Clock Up is kinda hard to blatantly defeat.

Diend's "Army" consists of 3 weaker than usual riders at a time.

Well, since I create these things, I'll put my input for each Rider throughout the thread.
Starting with DiEnd-
Most likely, he'll start off with Invisible. While Agito and Caucasus fight, DiEnd will FFR Agito into Agito Tornader and attack Caucasus with him. While Caucasus is distracted, DiEnd will summon J, and just have him stomp on Caucasus until he's unconscious. Agito, still the Tornader, will suddenly regain control and attack DiEnd's J till it fades away. On the ground however, DiEnd will have entered Complete form, and with all the movie Riders he summons (excluding Caucasus), will use his FAR and the finishers of the other riders to take Agito out.

Stay tuned for my input on Agito on this fight

Diend will only fight if forced. Let's say Diend went invisible (his most used power). Agito's Flame Form (and probably Trinity Form) can see him while invisible and Caucasus can Clock Up and run around until he finds Diend. Both can take Diend off guard easily. Diend, not being a fighter, would be taken out before he could even use his summoning cards.

EDIT: And Diend's army are knock offs that can be dissipated fairly easily. The Rio Troopers alone are normally taken out moments after summoned.

EDIT2: And I just realized that all of Diend's attack rely on his gun. One off guard punch from either of the other two would knock his gun out of his hand. Diend is now more useless than the Decade version of Kuuga. Bottomline: Diend's weaknesses are huge.

DiEnd. He'll just summon up an army of Riotroopers as a distraction, chill in a corner while invisible, then, when everyone else is tired, steal the Hyper Zecter, Hyper Clock Up, and snipe everyone in the head at the moment before they begin to henshin (as well as stop a time paradox by givving the DiEnd of the past the Hyper Zecter of the Caucasus of the past and instructing him to travel back and do the same).

Didn't TheBee take out 3 Riotroopers (The max amount he's ever summoned) in one hit each?

Also, everyone casually speaks of Diend taking Caucasus's Hyper Zector, when Diend is one of the few Riders who's had his Transformation device stolen while he was holding it >:I

Who's to say DiEnd can't summon more?

The fact that he never did.
Unlike things like Bashaa fever, which we can confirm exists despite having never seen it, because of Garulu Fever and Dogga Fever, Diend has never summoned more than 3 riders, and showed no ability to do so. He wasn't Basco summoning wave after wave of characters :P
His Gekijouban card did summon 9, but that was only as a part of Complete form, and we don't know if they have the same limitation as Decade complete (One Hissatsu before fading. No additional fighting)

The assumption that Diend summoning an army is unfair if he never did so.

I'm voting DiEnd. I don't remember if it's just Decade that can Clock Up, but if he can, then major advantage, against Agito at least. Plus, the whole summoning thing works for him.