1. Rider 1 & 2: As I've said before, the 'new' silver & red & striped versions to me ARE Kamen Rider, but the earlier suits are cool too. Color schemes aside though, they are basically perfect in every way: great belts, great helmet designs, just the right amount of armoring and slimness. They're the oldest and the simplest, but they're still the best. Every Kamen Rider designer should look to these guys first, then work from there. And no I will not pick between the two.
2. Skyrider: An utterly fantastic costume with the best helmet design in all of Kamen Rider. Like the Double Riders, I like the "New" version the best, though I've got a soft spot for the older darker green/brown one. What the new one brings though is that more leathery material used on the suit (particularly the arms) that just looks sleek as hell. Top it off with a great color scheme and the black gloves & boots (which are a reminder of his Neoshocker origins, as many of their monsters had those too) plus one of the best belts ever, and you've got a winner.
3. X: Went through a ton of changes, but I like what we got the best. At one point X would have had a lighter body suit with light blue gloves and red boots. At another, a silvery foil-looking body suit. What we ultimately got was the rather drab grey, black and red, but I think this works. Coming off of the more colorful V3, X-Rider instantly sets himself apart. It provides a nice contrast to Apollo Geist and actually reflects their mindsets (I dunno how intentional that was, but knowing Ishinomori's stuff I wouldn't put it past him.)
He has a great belt, a great mask (always liked the Perfecter) and that great scarf. Looks great in a team along with...
4. Amazon: Proving that Kamen Rider could go beyond the insectoid look and still work. Awesome body suit with the red lines that make drawing him a pain in the ass, but I wouldn't have it any other way (I've had like two decades to try, anyway.) Great helmet, great color scheme, and I love his belt.
5. V3/Riderman: I put them together because I like them equally, but I think they just look good together too. Every time V3 shows up, Riderman should never be too far behind. V3 is classic, taking bits of Rider 1 & 2 but at the same time bringing a lot new to the table. Riderman I like just for how unique it is. I might be in the minority here, but I think covering up his mouth is just about the worst thing you could do; it basically defeats the entire purpose of what the character is supposed to be about. Those arms rock too. Great colors all around.
Runners-up, in order: Stronger, Supes, ZX, Tackle, Agito (Ground), Kuuga (Mighty), Femme, Blade