To be sure, each of Romney’s finalists or near-finalists has merit. But Ryan would benefit Romney’s candidacy in unique ways. Here are just a few:

Picking Ryan would excite and unite the party.
By putting Ryan on the ticket, Romney would add the party’s single best spokesman on three huge issues: Obamacare, the budget, and the debt. Imagine Ryan debating Joe Biden — or Hillary Clinton.
By adding such a heavyweight to the ticket, Romney would convey to the electorate how high the stakes are in this historic election.
The pick would also show strength. By making it, Romney would (rightly) indicate that he’s not afraid of being overshadowed by anyone.