I'll be the oddball here. The two smoothest actions I've ever handled are an Argentine 1909 and a Mannlicher-Schonauer in 30-06. I own the 1909 now, but the MS was more than I could afford at the time. The 1909 is one that was imported as 7.65x53 and reamed out to 30-06, but with a 0.313" groove diameter. It shoots 180gr round nose .308" bullets pretty well, but nothing else. Its destiny is to become a fast-twist 6mm Remington later this year. At least that's what it told me it wanted to be.
I don't have nearly as much experience as many here, but I've handled quite a few guns in passing and those two still stand out to me.
Matt