You need to determine your budget, the size you think you'll need, and the features you want.
I have an Engel 45F combo fridge/freezer meaning you can put in a divider and run one side as fridge and the other freezer or any combination. I also have a ARB Elements 63qt, I believe it's the only one on the market that is rated to work in the rain/snow/heat on the back of truck bed. I recently purchased a Dometic CFX3 75DZ, that has two separate compartments with their own doors, this one allows for control of each compartment individually, including allowing to run one compartment and have the other turned off. I'll be selling the ARB as I'm putting a canopy over my bed. The Engel fits perfectly in the tongue box of my trailer.
I can say that the Engel is the most efficient in terms of power. I have high hopes for the Dometic, really like that I can turn one compartment off, if I'm not using it. I have a friend that has had one for about a year and she said it took Dometic a week to get back to her, but they sent her a new one under warranty. I can say that the ARB Elements does what it's advertised to do, it keeps on trucking in the rain, mud, and dust (haven't tried it in the snow) and it's the most quality built one I own, second would be the Engel, than the Dometic (however Dometic makes replacement parts easy to get on their website, ARB hasn't gotten back to me in over 4 months on a request to purchase a replacement 12v cord.