wow... I am quite surprised at some of the answers. I agree with Mysticplayer. The 243 would be at the top of the list. Shoots 1000m no problem where 223 and 308 would be stretching. 223 will do it but would be best served with a fast twist and heavy bullet which may mean custom barrel for 1000m. 223 is also great for varmints. 308 is overkill for varmints in most cases and have great selection of bullets with good bc for 1000m but many feel it lacks punch. 243 has great selection of bullets from varmints to deer to 1000m shooting. Only downside to 243 may be cost of ammo but if you reload then it shouldn't be a problem. Having said all that I have many 308s. Just love the round.
Remember when choosing a rifle and caliber to be realistic. I chose a Remington 700 with 26" heavy barrel in 308 to shoot out to 1000m only to realize that there are no 1000m ranges locally so it rarely ever gets used. I have a bunch of other 308s that are better suited for my regular shooting. Plus if I were to do it over I would choose 243 for 1000m (actually probably 6br).
Remember when choosing a rifle and caliber to be realistic. I chose a Remington 700 with 26" heavy barrel in 308 to shoot out to 1000m only to realize that there are no 1000m ranges locally so it rarely ever gets used. I have a bunch of other 308s that are better suited for my regular shooting. Plus if I were to do it over I would choose 243 for 1000m (actually probably 6br).