Manga comparison since I prefer manga a lot more than I do the anime and they are much more even in the anime.
Skill - Ranma appears to be a lot more skilled than Ryoga. Example in skill difference Ryoga was fighting all out trying to kill (where as Ranma wasn't trying to kill) Ranma while Ranma was in his weaker, less skilled (as shown by Mousse battle mainly from lack of experience fighting as a girl) form and ignoring Ryoga in favor of arguing with Akane and Ryoga still didn't win.
Toughness - Ryoga is tougher than Ranma but not by an overly large degree. Ranma did take a massive punch by Ryoga into a mountain side causing the rocks to compress and has taken vacuum blades and still been able to fight.
Ranma and Ryoga toughness comparison done by obsidian fox on fukufics.com before site crash:
Toughness => Power => Equivalent Fight
1x => 1x => Soldier (3000W) vs. Soldier (3000W)
1.13x => 1.2x => Soldier (3000W) vs. Fit Male (2500 W)... it would hurt
1.3x => 1.5x => Soldier (3000W) vs. Avg. Male (2000W) (he could take a bunch of them)
1.5x => 2.0x => Soldier (3000W) vs. Unfit Male (1500W) (if he swings really hard, the soldier might even feel it.)
2x => 3x => Soldier (3000W) vs. debilitated old man who can barely walk up stairs (1000W). (Please go die somewhere else...)
So... if Ryouga is only 2x as tough as Ranma, he'd be able to ignore pretty much any blow that Ranma ever makes... even if Ranma made a lot of them. 2x is really the maximum one could place Ryouga's toughness over Ranma's. 1.5x is a truly plausible value for Ryouga's toughness boost. Much higher than 1.5, it wouldn't matter if Ranma threw 10 punches or 100. The most they'd ever do is cause light bruises.
However, on the other end, 1.3x toughness might be a little on the low-end for the boost, unless you consider toughness in other ways (e.g. extra endurance, pain resistance, etc.). With much lower toughness, Ryouga wouldn't be able to pretend those blows didn't hurt him.
Endurance - Ranma has shown to have more endurance (ability to keep going even though exhausted or hurt) than just about anyone in the manga (Toughness and endurance are not the same thing). Ranma was hurt by Mikado prior to fighting Ryoga and was capable of continuing. Ranma was capable of taking multiple ki blasts and keep going. Ranma starved himself for a week during the martial arts dining and was still capable of fighting.
Stamina - unable to determine though both have massive stamina. Both can fight for hours on end provided they don't use extremely draining techniques as seen during the rhythmic gymnastics training. Ryoga spend lots of time walking and Ranma swam the sea of Japan.
Intelligence - Ranma appears to be smarter than Ryoga and a lot less gullible. Ranma uses Ryoga's gullibility against him repeatedly.
Speed - Ranma is much faster than Ryoga.
Strength - Most people would expect me to say Ryoga and I will, Ryoga is stronger than female form Ranma. As for Ranma's male form it's unclear Ryoga could be stronger but it's also possible that Ranma is as strong or stronger.
We've never seen the full extent of either Ranma's or Ryoga's strength. Both are incredibly strong. Here are the arguments that I've heard people use to argue that Ryoga is stronger and my counters to them:
Argument: During Ranma's first fight with Ryoga someone said Ryoga is monstrously strong.
Reply: That was at a point in the manga that no one had seen what Ranma was capable of so their statement means nothing for determining their respective strengths. It does show that both Ranma and Ryoga are super humanly strong though.
Argument: Ryoga is stronger because he is able to wield an extremely heavy umbrella that Akane had trouble lifting.
Reply: The same umbrella Ranma's weaker girl form had no trouble lifting, swinging around with one hand, and jumping to a rooftop holding.
Argument: Ryoga accidentally destroys things with his strength therefore he must be stronger than Ranma who doesn't accidentally destroy things.
Reply: Ranma has accidentally destroyed things with his strength the boat that was going to be taken to China during volume 37 is one example. That doesn't prove that Ryoga is stronger than Ranma, it does show that Ryoga does not have as much control as Ranma does. People who are clearly much weaker than Ranma and Ryoga casually destroy property as well (Kuno with his sword, Akane when she is angry, and Shampoo just likes doing so as a show of strength).