Arabia isn't bad in MP; rushing Mamluks that don't require iron in 3 techs is ridiculous
Well the obvious examples would be the naval civs like Norway and England since people only play pangaea in MP pretty much. France as well with how their diplomacy bonus works which is more useful vs the AI than vs other human players. I would also argue the Aztecs because they're stronger on slower game speeds since you can make more use of Eagle Warriors before they become obsolete (as you have more time to move around the map), and people only play on quick or online speeds in MP where this is much less effective. Aaaand also any civ aimed at Religious Victory since that is currently impossible in MP since you can just kill apostles with military units if need be, so Spain, India and Russia will also be better in SP because of that. Speaking of Russia, Peter's Grand Embassy bonus is obviously more aimed at high difficulty SP games (especially Deity) rather than MP. Also Kongo will be slightly better in SP because the AI will actually spread their religion regardless of who you're playing as, whereas in MP they can just ignore Kongo altogether
Sooo in summary I'd say
Better in SP/worse in MP:
Aztecs
England
France
India
Kongo
Norway
Russia
Spain
More or less the same in either:
America
Arabia
Brazil
China
Egypt
Germany
Greece (either leader)
Japan
Rome
Scythia
Sumeria
Can't really think of anything that'd be stronger in MP than in SP though