When it comes down to overall ability and, as of right now, how things look in the long run, I'd prefer Dolph Ziggler. I'm not going to start hating on Ryback just because he's a big powerhouse. I know there's a pretty substantial number of net fans that believe that you can't be a good pro wrestler if you can bench press 500 lbs for some damn reason.
Ryback is someone that I think is going to go far and that it'll have generally more to do with the WWE Machine behind him coupled with his look & general intensity. Ryback physically looks like a dominant beast and it's always been relatively easy for most fans to rally behind guys like him if the company really promotes the crap out of them.
For Ziggler, his success has revolved mostly around the fact that he's just flat out great inside the ring. Ziggler is also probably the best seller in wrestling right now. Within the past year, Ziggler has developed himself well as a character and has shown himself to be strong on the mic, so now he has the personality to go along with great in-ring capability. Not that I don't think Ryback is any good in the ring but, let's face it, he's not in Ziggler's league just as Ziggler isn't in Ryback's league when it comes to the look.
I'd personally prefer Dolph Ziggler but I have a feeling that when it's all said and done, Ryback could possibly be the bigger star. To me, it seems that WWE is conflicted about Ziggler. On one hand, Ziggler is loaded with overall ability and has worked extremely hard the past couple of years to show that. On the other, Ziggler is phenomenal at making other wrestlers look like a million bucks while looking good himself. It's a role that he's excelled at on a level that, in my eyes, is pretty much on par with that of guys like Christian or Chris Jericho. When it's all said and done, I think Ziggler will be on a level similar to Chris Jericho. He's someone that fans will enjoy & respect for what he brings, and he'll probably frequently pop up in the main event picture while ultimately hovering in the upper mid-card scene, but I don't believe he's ever going to ever truly be THE top guy. Ryback is someone that, in some ways, reminds me of Batista in that he has a killer look and great intensity. In my opinion, Ryback is far more entertaining inside the ring. He needs work on the mic but, for the most part, I think he's better than Batista on the stick as well. He's got the look, the intensity and the fans have rallied behind him over the course of his feud with CM Punk, whereas it's really taken Ziggler years for that to happen, at least without Vickie Guerrero. The jury is still out on Ryback in the long run but I have a feeling that, if they can, WWE try to ultimately put Ryback on that upper most level.