That's a tough one, depends on what one thinks of as "best". The one that I find best in terms of fun is the M-4 I have for work, my Mossberg SSi-1 single shot is best for a knock around gun, and the Remington 700 VS SF I have on order I feel is the best for its intended use. I like the Mini-14 also, it is fine for its purpose though it seems to be not overly ammenable to accurizing, at least to the degree of the ARs.
That is a bit like asking what is the best tool to put in a tool box but an interesting question nonetheless. I'd rather have multiple tools than only one tool anyday! Same applies to firearms. My current tool is a 12FV heavy barrel which I really enjoy but someday I hope to add an AR-15 to complement it since the AR just plain looks fun to shoot. The 223 is a great round IMO for target shooting and plinking and ammo cost is not as big of a deciding factor in how times I can take the 223 to the range.
I have an HK SL8-4 and love it. It is said to be one of the most accurate semi auto .223's out of the box and while I can't say that for sure, it is quite accurate. I hear the Swiss Arms PE 90 is more accurate but costs alot more also. This past weekend I was shooting gophers with it and have no problem hitting them out to 250 yards or so. It was a blast. I have about a thousand rounds through it so far and it has functioned flawlessly so far. I also like the fact that due to the gas operation design it does not get very dirty and is a breeze to strip and clean. I also like how easy the barrel is to clean because I never have a problem with copper fouling due to the smooth chrome lined bore.
Bringing this thread back to life just because it's a fun topic for discussion :wink:
Since I'm here, I'd have to go with a scoped, bedded bolt-action with a free-floated heavy barrel and a bipod. That would be one fun rifle. As a matter of fact, I may just have to make that my next rifle!
The AR is definately a worthwhile platform, and capable of some scary accuracy, but if I was gonna have only one .223, that wouldn't be it. If I already had the one mentioned above, I could definately see an AR carbine with a free-floated barrel in my future.