To start, competitive battling isn't just about using your favorite Pokemon to battle; it's about focusing on your team's strong points to win a battle. For example, Ferrothorn has high defenses, a good support movepool and a good defensive typing while Hydreigon has high stats all around and a wide offensive movepool to take advantage of those stats.
But it's also about synergy. 1 Pokemon can't just take on a team by itself; that's why other Pokemon on the team who can cover its weaknesses are important.
Each Pokemon belongs to a tier: OverUsed (OU), UnderUsed (UU), RarelyUsed (RU), NeverUsed (NU) and Ubers, based on their usage in OU.
I contacted PlatinumDude for you! :3 and okayy I understand people tend to get busy, so that's fine! Just please inform me if you feel you no longer need a mentor in the future so I can take you off to free space. :D