If I were you I wouldn’t focus on a particular element so much. The majority of the time you will pick a monster for its moveset rather than its element. Link strategies are a thing… but only once you have lots of monsters and have enough that require link and make it worth building your team around that.
As a general guide for which monsters are good you should look at the Neo Monsters current tier list thread. That splits it into General (PvE) and PvP. A lot of people focus on PvP so I’ll give you a quick idea of the elements seen in the top tiers of those:
S+ tier = 2 earth, 1 fire, 6 holy, 2 shadow, 3 storm, 2 water
S tier = 1 earth, 1 fire, 2 holy, 5 shadow, 5 storm, 4 water
A+ tier = 3 earth, 4 fire, 3 holy, 4 shadow, 5 storm, 2 water
Total = 6 earth, 6 fire, 11 holy, 11 shadow, 13 storm, 8 water
I hope that backs up what has been written by me and iSegal. Storm has a lot of good quality monsters and Holy stands out as one with very powerful ones.