For 7" senkos I use 5/0 round bend offset shank gammie worm hooks.
These: http://www.tacklewarehouse.com/descpage-GRBOBR.html?from=calfish

When you select senko hooks, you should consider how you set the hook. If you set with super hard hookset, the round bend is your best bet IMO because the angle of the hookpoint and the fact that the hookset will chop the bait in half. If you let the fish swim with the bait and do more of a reel set, EWG is probably going to work better for you. The dynamics of this are a bit hard to explain, its just that when I think back to all the missed bites I've had on these type of baits, this is just how things have played out for me.

Picking the right hook for any bait has to do with the length of the lure, how the fish eats the lure, the position of the hook relative to the weight (if any), and a number of other factors. If you want to experience the highest hookup ratio possible for any soft plastic lure, you should always experiment with hook sizes and styles to find the combination that gets bit the best and results in the most hookups. I hear people say that they only use EWG or they only use round bend or they only use straight shank. If you think like that, I think you're limiting yourself because every bait size, shape and style has a best fit hook wise, and its not always the same style hook.