took the drain plug out of my boat(thought I had a leak),what is the best sealant to us when I put it back in?

Did you remove the actual drain plug housing, or just the drain plug itself? Drain plugs shouldn't need any sealant. If it's a screw-in plug, there's often a captive washer involved, which is why no sealant is needed.

removed the housing

Marine "below water line" clear silicone.

I have had best luck with 3M 4200 marine sealant.

Academy sells "marine silicone" in a toothpaste like tube, but says not for below waterline use. It's not easy to find some that actually does. (Apparently the manufacturers don't like to say for below waterline due to legal concerns.) Don't use 3M 5200 Marine Sealant - it's just too much, very messy, and can pull gelcoat off if you ever need to remove the part again.

Unless you keep the boat in the water a LOT, I believe the above marine silicone will suffice. I've used it and it has shown to be VERY durable. Did you pull the part because you thought water was leaking around it? If so, next time, check that theory by putting water in the boat with the drain plugged while it's on the trailer, then watch for a drip outside the boat. Doing this makes the leak go in reverse so that you can isolate its source.

There are few applications that any type or grade of silicon is suitable for. 3M 4200 is a sealant, adhesive, and can be easily removed if you ever need to reseal. Also, when removed, silicon leaves a residue that nothing will adhere to including more silicon. All in all, it's some really awful stuff.