Kodachrome was always the finest grain, sharpest tranny film, because it was thinner. Essentially a black and white film, was made without any colour dies, only filter layers and silver layer- dyes were added at processing stage, with correct dyes migrating to the filtered layers. This process had something like 14 steps, with absolute control required, hence only Kodak and a handful of others could successfully process it. I stepped in during the changeover from E-4 to E-6. This was a great improvement in small lab processing, but until Velvia, no E-6 film could match the fine grain of Kodachrome.
Presumably this was made possible by the relatively recent improvements in grain technology. I've not tried the Rollei, or some of the other new ones, but with so much difference in the actual structure of the film, it's hard to imagine ever getting close.
(Also, Fuji were the first to achieve consistency from batch to batch,- a revelation in the eighties, especially with colour print paper).