The original Rider Double Kick is a great one, and I think I don't even need to mention it.
For other finishers:
Ichigo's Tenko Rider Kick. From the Seigi no Keifu videogame. It seems like an extremely powerful move, add that the guy who's doing it. You can't survive this.
Nigo's Rider Swirling Kick. He came up with it in order to defeat an enemy who had bested him in combat. And then, that's one of his most powerful finishers in the aforementioned videogame. I think this is very powerful too.
V3's Mach Kick. A kick in the sky to an enemy mid-flight is always great.
V3's Hanten Kick. Awesome, just too friggin' awesome, I'll never get tired of seeing it.
X's Hell Wheel. It just has to be seen.
Amazon's Dai Setsudan. You either bleed a lot, or lose body parts. Perfect
Any finisher that Stronger does in Charge Up Mode. The electricity it gives to the enemy is overwhelming, which makes you feel it more when they blow up.
I haven't seen Skyrider, but I've heard he has arguably the best finishers of all Riders, what with the 99 Techniques and all that.
ZX's has his own share of Finishers in Spirits, most of which violently murder the monsters. I like them.
Black's Kingstone Flash is pure awesomeness. Even when not used as a finisher.
Black RX's finisher with Revolcane, likewise.
Agito's kick in either his basic or Shining Form.
Every finisher in 555 is sexy.
Blade's Royal Straight Flush.
Most of the finishers in Hibiki.
Kabuto's Rider Kick. Not bad at all.
Any finisher of Decade that does not involve a Final Form Ride.
Double's finishers as FangJoker, CycloneJoker and Xtreme.
Every finisher of OOO.
And last but not least, Cyclone Attack and any other finishing maneuver that involves a motorcycle.