At this moment, a S&W M-15. Because my M-18 needs company and the M-19 isn't quite exactly right.
Also on the list are a 4" S&W M-28, just like the revolver I learned to shoot handguns on. And I would be remiss if I forgot to include the models 36 and 37. Actually know where there is a clean looking 37 for sale right now, it's just not a priority on the list. Saving the cash to pounce on the next clean M-15 that turns up.