On topic, I too have to go with The Rock. The guy is in the absolute best shape of his life right now and can clearly still go with the best of them in the ring and on the mic. A close second is Brock Lesnar, but I feel that his whole persona relies on making an appearance every few months and if he were to become full-time, he'd get lost in the shuffle eventually. Also, the guy's a megastar nowadays so he'd be great business wise.
As for how I'd book him, I too would go with Rusev as The Rock's return feud given the history they already have together. A good two month feud with Rock ultimately getting the better of Rusev. From there, I'd have Bray Wyatt target The Rock for one reason or the other leading to John Cena coming to his aid. This would lead to a tag team match at EC pitting Rock/Cena vs Wyatt/Harper with Cena/Rock getting the win. Next night, we get the mega match of Cena/Rock for the #1 contendership with Rollins ultimately screwing The Rock because he doesn't want to face him. This would lead to a triple threat match at the next PPV with Rollins retaining. Tensions between Rock/Cena escalate following the match and in the next few weeks leading to a match at Summerslam with Cena coming out on top. At this point, he'd be primed for a heel turn. After a short feud with Dean Ambrose, Brock Lesnar returns to challenge Rock at Survivor Series, where we get a Lesnar DQ victory after Rock beats him down with a chair, continuing the feud. At Royal Rumble Lesnar finally pins The Rock sending him over the edge. With two men left in the Rumble (Reigns, Rusev) Rock comes from under the ring and Rock Bottom's Reigns leading to his elimination. After 2 months of taunting and avoiding Reigns, calling him a failure, a nothing, a disgrace to the Anoi'a name, etc. they finally meet at Wrestlemania. Reigns gets the big win.
This was completely off the top of my head and although there are some sketchy parts, I could see it working well enough.