Love the airlines in Star Alliance. Live in Canada(ACA A Star Member), go to China(CCA - A Star Alliance Member) frequently, transit through US(UAL - Star and others) all the while using Miles and More(DLH - Star). Also the way Star Alliance connects the stars of the world(double pun intended) in a great way.
OneWorld is small but still gets me where I need to go most of the time. Small selection of airlines, but still lots of destinations. Use the Finnair Plus card.
SkyTeam has a great livery and I mean like really good. but the rest sucks. Unless flying domestic in China, I don’t fly them. Little coverage outside of China without connecting. Also, the airlines don’t cooperate with each other. I use a KLM Flying Blue card because I like KLM the most out of all SkyTeam members and flying blue has a partnership with WestJet based in Canada(again, I live in Canada).
In summary, SkyTeam has best livery, Star Alliance is the best airline Alliance and oneworld is just mediocre. And yes I do have frequent flyer cards for all three Alliance and more than the ones mentioned above including(ACA CPA AAL BAW).