>>39319192What for? Hardcore competitive, friendly competitive, or just for story?
If A. then you will probably be fine for at least a while by just picking six of the top twenty most used mons for whatever format you're playing and using the first set that shows up on Smogon.
If it's B, ironically, might have to think a little bit harder on how to make a good team. You'll need to take a look at what mons you like, decide which ones are viable, while still trying to make sure your team is cohesive (e.g. you wouldn't run chansey on a team with 5 other mons that are hyper-offense). Try to see what qualities certain mons lack and find another mon that fills that gap.
If it's C, you will probably do fine by just making sure you don't stack too many of the same type on your team and avoid using mons with abysmal base stat totals that make them hard to use even at a very casual level (like Ledian).
