First off, Shaq wrestling at Wrestlemania won't be a pretty sight no matter who he's facing. There's really no getting around that. It'd be way better if he were somehow just involved in a match as a manager or something along those lines, sort of like Mickey Rourke's appearance a few years back. But, if he were to wrestle, it'd probably either be against Mark Henry or Big Show. Now although I'd rather see him verse Show, I think it would actually make more sense for him to verse Henry. Why? Because Shaq would be the natural face, and I think Henry would be the more sensible heel for him to face, not Show (who isn't even a heel right now). What beef would Show have with Shaq to actually draw some heat as the heel? Just for some revenge from a few years back? I don't know, I think Show is too much of a face right now to go down that route, whereas Henry could step right in and involve himself in a classic face/heel feud with Shaq right away. Plus, it seems as though they are trying to set up a Rhodes/Show match for 'Mania.
So, in conlusion, although I'd rather see him verse Show, it'd probably make more sense for the WWE to go with Henry. But I kind of have a feeling it'll still be Shaq/Show at Mania for some reason.