I think by default people choose instances related to their region. That way you can communicate with likeminded people in the timeframe they’re the most active.

If not that then you look at the topics instance specialises in. For example:

• mander → science
• gtio → debate
• fapsi → creativity
• slrpnk → solar punk
• hackliberty → tech anarchism
• heapoverflow → programming etc

full list

If you don’t care about specialization and prefer simply more content you choose general instance such as beehaw or sopuli