Which pick, if called at #56, would just eat at you and make the rest of the draft difficult to enjoy? It needs to be a pick that is at least faintly plausible.
For me, it would be Montori Hughes at #56. It would be such a classic first pick by Seattle. Hughes may not have any pass rush ability, but he's a legit athlete and has incredible core strength. He's a raw high upside player that would play DT in 2013 but would probably take over for Red Bryant down the road.
Taking Hughes that high would also be a monster reach, but I don't think Seattle will care about that if they like Hughes as much as I fear they might.
Jonathan Hankins at #56 would suck balls as well. Bennie Logan would be unpleasant, but at least with Logan I see Alan Branch type upside to him, so it wouldn't feel like a wasted pick.