I don't know if "its the best", but I use DAP clear flexible sealant. It's available at Home Depot and is about $8 a tube. It's a polyurethane base (no silicone) and works well. It's difficult to push through a very small opening (I cut just the very tip of the applicator tube), so I bought a pneumatic caulking gun from Amazon. 35 PSI, easy squeeze on the trigger and it lays a 1/8" bead that's easy to tool without smearing the bead. With a hand operated caulking gun I could not apply a thin enough bead, so I was always "wiping away the excess" and smearing the rest.....
Here's the gun I bought, $27 well spent, in my opinion:
https://www.amazon.com/dp/B000LQN5G4...oding=UTF8&me=