The wattage of your microwave will - outside of unique model-specific technology - largely determine how effective and quickly your microwave will cook food. A good baseline to work with when choosing a microwave is 1000 watts. This will ensure that your microwave will be able to perform most functions optimally.
Having a 1000 watt or higher microwave doesn't mean that your cooking options are limited. For example, meals that require "medium" heat aren't now off-limits, higher power microwaves can also come with adjustable power. If you're used to cooking on medium heat or prefer some versatility, selecting a higher-wattage microwave with power options is your best bet.