Best heating pad for indoor bunny

Why would you need one? They like the cold. If you don't have to wear a winter coat indoors, then your rabbit won't need any extra warmth either. Anything electric like that would risk bunny chewing the cord and getting electrocuted.

Snugglesafe. They are microwaveable and specifically made for small pets. Though no way to know if your bun will like to sit on it.

I had an old bun that was arthritic and had hind limb paralysis, and she needed extra help keeping warm, plus my rabbit room was in a colder part of my house. So I rigged up an electric space heater set on tile on low, just on the outside of their secure play pen fencing. I placed the heater close enough to blow into the pen over her usual spot to lay and sleep, but far enough away from the bars that if she scooted too close to the fencing panel, she couldn't get too close to get overheated. The most important thing with that was making absolutely sure my rabbits in the pen couldn't manage any contact with the heater or it's power cord, as well as having a safe non flammable space around the heater itself. It was also very important to check the filter daily, and clean if needed, as it could be a fire hazard if it got clogged up with fur. And not leaving it on unattended. I'm not recommending using a space heater, there are potential hazards with their use. Just saying what I did and that it worked well for my rabbits and my situation. My old bun was always wanting to lay there near the heat, so I know she liked having it.

There are pet heating pads, but no idea how rabbit safe they would be. They would have to be safe from chewing and no way to chew the cord.