Best is very subjective.

.world is a good general purpose instance for just about anything. I think it has the biggest population at the moment, so communities there are likely to get at least some engagement.

For “general discussion” it doesn’t really matter. The instances are federated so you’ll likely get general discussion in comments from lots of people from lots of instances anyway, wherever your community is based.

Some people get almost nationalistic about their chosen instances or have grudges against people from certain other instances. There’s sometimes inter-instance politics with some servers defederating with others or threatening to for various reasons. It’s kinda fun to watch in a popcorn drama kind of way. For the most part, the instance doesn’t matter.