I'd go AR-15 in .223. It's the semi-auto with the best cost to accuracy ratio. This will probably require the least amount of gunsmith intervention. You could build from parts and save a few bucks. Depending on what you want such as barrel length (16, 20, 24, or something else), handguards (standard or straight through), trigger, barrel manufacturer, it may just be best to build from parts.
If you want to have some real fun, go with an M1 Garand. Get yourself a rack grade Greek issue from the CMP, have it reworked by a real gunsmith and get yourself a nice .308 Krieger barrel for it and a DGR stock. People will recognize the righteousness.
M14s are nice too, but it will cost a lot of money to get them anywhere near as accurate as an AR. We're talking about getting a good receiver from someone like Fulton Armory and rebuilding with milspec parts. Go with a Kreiger barrel and send it to McMillan to get it bedded into one of their stocks.
Now that I think about it, I really feel like rebuilding a Garand...