Tough one, I own 8 which I feel all have legitimate good aspects even XMLE and PotC, plus I sold SM last year so call it 9 solid Stern pins I've owned in the past year still kept 8. I'd say SM was sold because it was close to STLE but either are great.
I'm kind of torn because I want to say Metallica. It's got the whole enchilada, nice ramps, music, art, rules, toys it's the Mac daddy.
However I'm also in mind that ACDC premium is nearly as good minus the good artwork and STLE is stunning to look at and play. IM is super fun and Tron is nearly perfect.
Can't forget Tspp whose rules are as good as any plus great call outs.
I'm leaving XMLE and PotC for the lowest tier but think they're both excellent pins.