Both are really different.

In Mp there's a bunch of mods, you can be good at Aikido but you can suck at Wushu. Also there's a lot of people, you can never know if you're the best or not. Getting in Mp isn't easy, you have to play a lot of matches to get at least a bit of knowledge of what you're doing.

In Sp, each player has his own style, there's a lot of ways to play it and each one requires so much knowledge (madmans, tricking, parkour, solo-spar).

The thing is, both things aren't really comparable, they are different.

Even though they are different you can still say which one takes more skill to be the best at it. I think it would be easier for me to be the best in the world at giving haircuts vs sculpting. Two different things but I can still say which one takes more skill.