Matte or gloss? It's really a matter of preference.
However, for the sake of your health, and that of the environment, it's best to use an organic varnish (made from natural products) which can be bought in organic outlets or online.
At the very least, buy a varnish that's green seal-certifed meaning that its VOC (volatile organic compound) content will be very low (compare with other big name brands of varnish that are put to the fore in hardware stores, you'll be surprised!).
Be careful, varnishes produced by brands specializing in arts and craft products may be just as harmful. Avoid solvent-based varnishes (that are washed off using white spirit rather than water) such as the old kind of Pébéo varnish, which was an attractive amber color, but which gave off a very strong, unpleasant odor even long after the object varnished had dried.
You should always apply varnish in a well-ventilated room, away from children and pregnant women. Wait for a few days before bringing the varnished object or piece of furniture into the house.