Nowadays there is a big selection on the market you can choose from. There are two, maybe three things to consider when buying one. Again, the same with ropes, harnesses, and shoes, … the main decision-making factor will be the type of climbing you will use it for and we will talk about it a bit closer below. Another factor should be its handling characteristics, how easy and intuitive it is to use, and how it actually feels in your hand. This can be quite personal and the best is to try it out before buying. The last thing you could be considering is its versatility as some of the belay devices can be used for single or double/twin ropes and also for abseiling. One could also consider size and weight but that is not that important.
Belay devices suitable for sport climbing
When sport climbing we use a single rope. When working on some hard projects we do spend some time hanging on the rope trying to figure out the moves and then practice them to find the best beta. For this, the best is the belay devices with an assisted locking mechanism which locks the rope when weighted by the climber which makes it easier for the belayer to hold the climber up. All that needs to be done is to secure the end of the rope with your hand to avoid any failure.
Most people are using Petzl GriGri which works very well and it has been on the market for a long time. Personally, I would stay away from GriGri + as it is a pain to lower climber down with. Last few years other companies have come out with similar devices but they are not that convincing.
The very good and easy-to-use system we like a lot has these three quite similar devices. Quite small, light, and simple.
Belay devices suitable for multipitch and trad climbing
Obviously, there are gonna be bigger demands on versatility when doing multipitch or trad climbing. You definitely want to be able to use single and double/twin ropes and to be able to abseil with the same device. A very good thing is also to be able to belay direct (connected with the belayer body) and indirectly (the belay device is connected only to the anchor system). Here we would say the best works classic belay plate from Black Diamond ATC GUIDE or Singing Rock SHUTTLE. If you would prefer something with assisted locking mechanism although here the use is not that big Ederlid GIGA JUL is a nice option.