There isn't a single monitor that will do well in that size enclosure, there is no way to provide the husbandry requirements for them. What are the dimensions of this tank? There are a few questions you can ask yourself to make a decision. Do you want something that eats insects? What type of temps are you able to provide in that type of tank? Are you wanting something that is always hands on? Lizards generally require a lot more care than a snake, skinks are not "brats", however they are fairly intelligent and some of the Indo ones that come in wild caught need to be worked with to tame them down.