PokéQuest is probably the best Pixelmon server I know. It has amazing staff members, events, mini-games, a competitive Pokémon Battle mode and welcoming community.
I found out about the server thanks to a friend who has played on it for a while. The first time I joined the server about six people I've never seen before welcomed me and were very helpful and showed me how stuff worked around there.
I do really recommend playing on PokéQuest for the best pixelmon experience!
(I do also recommend joining the Discord server where we chat and play different kinds of games all the time!)