Stern, Spooky, JJP, etc...all have the potential of making great games...they can also swing and miss. They all have their strengths and weaknesses.
Pinball can be great and fun and made in all kinds of ways. I think great games happen when the total package just works and comes together.
I rather focus on their strengths, so here is my view...
Stern - capable of: cool custom mechanical features and toys, great artwork, fast/flow, "snappy", great code, make a lot of games, great themes, a variety of designers and teams, market pinball to the world
When they hit on all cylinders, I think it's the best experience in pinball.
Spooky: - limited numbers of games that help hold value with supply/demand, best playfields, customer service, best light-show when Scott is doing it, best speakers. They're not afraid to take chances and have edgy themes/callouts etc... Just great people that you feel awesome about supporting.
JJP - great LCD animations - the most rich in pinball, great sound, the deepest code in pinball generally, really push mechanical action and innovation, "packed playfields"