If you consider an AR15 milsurp, which it isn't but it is a military rifle, then without a doubt an AR gives the most features, most useability and most option to upgrade in the future.
Entirely depends on budget and what you are looking for.
If you want historical value, then a Lee Enfield, Springfield or K98 Mauser would suit you better. If cheap thrills is your thing, then a Mosin Nagant.
Cheap ammo for plinking would be a Mosin, SKS or an AK47/74.
I'm not sure what the price of ammo for 223 is now, but for an accurate, fun, nice and easy to shoot rifle, for a reasonable price and historical value not being important, then I'd say AR15.
Personally if I wanted a milsurp rifle, I'd want something with historical value, and something I would accept I had to reload for. I'd go for a Lee Enfield, or if I could find one a Swiss K31.