What does everyone use on heavy duty zippers to keep them operating smoothly?
The ones on my boat are getting super hard to move .

Silicone spray works, especially on nylon zippers, I usually just use a lanolin base spray on zippers, snaps etc.

Im a wax candle or plain parifin wax guy. Have used it for years on my dive gear and tent zippers. Works good on extention ladder rails tracks on roll up doors places you don't want oil running. . Warm up a squeaky hing melt some in end of problem. I've done that on piano type hing. Gotta make sure it's the hard parifin wax not the sticky bees wax type. That stuff picks up to much dirt grit make problems. If your get bit to much wax in one spot. Quick hit with old tooth brush usually good to go.

Mac

I've used old candles stubs before too, but it seemed to flake off too soon, maybe warming it up would work better?

If they are metal a pencil. Its granite. And on plastic & nylon spray silicon works great. Spray silicon is in the cans that are labeled wire dry. Cause its cheaper than the one made for mechanics to use on your car door hinges.