An oil-filled one is what I'm currently using in the rooms that still don't have heaters in them. It looks like a little radiator. I got it at reno-depot for about $60 and it heats nicely.

I also have a little square blower heater if I need an extra blast somewhere. I used to use it in my office while I was working until I was somewhat careless and had it too close to a bag of computer stuff I had just upgraded from, and the heater melted the bag and some of the computer stuff including a docking station and mouse. Yikes! So good heat, but loud and needs supervision.