i'm wondering what people consider the flattest shooting cartridge out of the 7mm/08, .308, .260, 6.5X55 ?? all replies are welcome

Flatness of trajectory and accuracy are NOT THE SAME THING! None of the rounds you listed is any flatter shooting than any of the others, for any practical purpose!!

Let's compare the cartridges you have listed. First the 7mm-08 and the 308, the 7mm will be slightly flatter but hardly noticeable, it's a 308 case necked down to 7mm. The 260 and 6.5x55 are just about the same thing with a slight edge to the 6.5x55. Having a flatter trajectory is not necessarily more accurate. For game up to elk any of the four listed will do what's asked of them. For factory ammo availabillity I would go with the 308 or 7mm-08, the 260 is becoming more popular but not readily available, the 6.5x55 is way underloaded in American factory offerings.

I don't really know which one could be loaded to be the most flat...but I would suspect it would be the 6.5 X 55,
And I don't know which one could be proven to be the most accurate...but I suspect the 6.5 X 55 would be in the top 2....it has been winning european and world shooting competitions (along with other calibers) for years.
If you don't mind the longer receiver then I'd pick the 6.5 X 55. But it would be hard to argue with picking the 308 b/c of the ammo availability............ACTUALLY, IT WOULD BE HARD TO ARGUE WITH CHOOSING ANY OF THE CALIBERS YOU MENTIONED...I have a 308 and several 6.5 X 55 but would love to own a 7mm/08 and like the idea of the 260 (almost as much as i like the 6.5 X 55!)

tough call... i think i would take the 7mm-08 or the 308 just because of ammo availablity though.

i can only chime in for the 308win

i have shot 150gr. - 165gr. ( that is all i need for deer in maine ) in several differant guns chabered in the 308win.

i have come down to find that the federal premium or ( vital shok ) 150gr. nosler ballistic tips shots the best at 100yds . ( under an 1" ) now for hunting ( just in case i come across a bear during the deer/bear season )i have found that the winchester power points got roughly 2" and the best was federal vital shok 165gr. sierra gameking's soft point boattail.

also works great for shots under 50yds. best initial damage i have seen in many ammo i have used for deer.

I don't know which would be the best. My 06 has always shot flat enough for me, but then 200 yds is rare for me here.