I don't know "best" - I just post the most
I can tell you that many people here have warned about using spray foam. I've never tried it, but I'm not sure that after seeing how a "good idea" from decades ago has turned into failure that I want to use something I can't remove again easily. There's also mention that some foams can create overpressure (and the surface area of a seat tank is so big, I can see how even slight pressure may generate more force than the seat can hold - one of the reasons, incidentally, to be really cautious when trying a pressure test to find leaks).
Pool noodles are recommended by many here. You can get the ones without the central hole to minimize air pockets. However, I think the new iteration of building foam (the pink or blue panels) should be sufficiently less susceptible to becoming waterlogged that it may be used (if monitored). The difference to the white foam is that there is no longer an "open network" of spaces between the little expanded pellets. However, even the blue / pink boards aren't proof against water vapor. I went ahead and filled my bow tank with it, because it was something I had on hand (and didn't know where to best source pool noodles).
It's been long enough, I should probably check...