Email program with best sponsor content?
 

........

You mean for promotion?
CECash
Gamma and
Freepassbucks - $1/email will let you choose their free content for paysites, when you promote the mailer programs ... The latter is backed by the reputable
Platinum Bucks and
Discount Pass Bucks - $25/$10 membership for the surfer

If you mean content inside the meber area, I guess no mailer has much content inside, the idea is to have upsells ...

WEGCash has a shit load of content you can download to promote them