First - check your laws. Some of them may not even be legal, narrowing it down nicely.
I like tegus - they are hardier, and easier to feed.
This whole group (monitors and tegus) is very dependent upon frequent, calm handling, if they are to be handleable. There will always be some wild ones, and some calm ones, on top of that.
If you are looking for a pet you can carry around, etc, I would either choose something different, OR buy an older one that is already used to being handled.
Keep in mind also that like with dogs/cats, some of them just don't like certain people. I know it sounds silly, but it's true.
I would definitely spend some time with each BEFORE getting one - so you can get a "feel" for each first.