My most-used filters are CPL and 10-stop ND, which I have in 77mm and 82mm. I also have 6- and 3-stop NDs in 77mm. I like that the RF 14-35, RF 24-105 and RF 100-500 all take 77mm filters, it’s a great travel setup. I'd never tried a vND until getting the Canon drop-in adapter, which I use mainly with my TS-E 17/24 and EF 11-24.

The drop-in adapter is very nice, but you also need the clear insert unless you want to swap out for the vanilla adapter when you don't want a filter. There are 3rd party covers, but the drop-in adapter is very slightly longer than the vanilla adapter (to account for the glass thickness) and that means unless there's a filter in there infinity focus is lost (at least, it is with my TS-E 17 – perhaps other lenses focus further out).