So I like having multiple mons so I can make chances and have fun. My Master team is limited as I play alone so I do not have legendaries. These are my choices.
Great League
Azurmurill B/PR/IB
Whiscash MS/B/MB
Munchlax L/BS/B
Flygon MS/DC
Macargo E/SE
Breloom BS/GK
Marshtop WG/S/S
Still need Altaria and possibly Umbreon
Ultra League
Lapras IS/IB
Steelix DT/C/E
Venusaur VW/FP/SLB
Breloom C/SB
Sceptile FC/LB/E
Electrive TS/WC
Charizard FS/BB/DC
Feralgatr B/HC/IB
Aggron DT/HS/SE
These are what I have and use and have pretty decent results. The hardest part is figuring out when to use shields and switch out mons. Others will be better with Master league but for that I almost always use B/C Tyranitar and BP/MM Metagoss. Have fun and enjoy. Hope it helps.