Meta Knight is obviously the best partner any character besides himself can have. He can tank for you or be the aggressor and he does well in 2 v 1 situations. Picking a partner up for Meta is not too difficult at all since his best partners are in the high or top tier (Those being the most common anyways). The difficulty in choosing a partner comes in the strategies they involve. Each partner brings unique strategies.
Game & Watch- Essentially, this is a team that focuses on Abusive Priority as Meta Knight has transcendental Priority (Nothing clashes) and G&W simply beats out almost everything. He's one of the best doubles partners only if the player is any good with him. Considering his lightweight hurt-box, the player must be agile and try to avoid being caught between everyone fighting. He's an awesome partner because he can absorb projectiles (Doubtful as projectiles are hardly ever used), kill faster than you and can brick wall with you by jumping above and using N-Air while you F-Air. The biggest plus is that it's next to impossible to gimp him so the only way to get him out is to rack up damage and actually kill him. Plus you have your own version of the Ominigay by using U-Throw and having the G-Dub Up-B at the top.
Wario- This team focuses on Aerial Mobility to control the stage. Meta Knight moves soooooo slow compared to Wario, which alright because you make up for each other's weaknesses. His Hax aerial movement is useful for pressure strategies and allows him to jump in and out of any situation. For example, Wario can D-Air someone while you can keep up the attacks to break the shield or at least shield poke. Wario makes up for your bad aerial movement and kill power while you make up for his range and attack speed. Wario is also not easily gimped and has more kill power. His waft adds psychological pressure on the opponents and gives them another thing to worry about which is a huge plus. This team's strength is being able to stay mobile the entire time allowing attacks to be avoided easily. It also means that your partner won't be yelling at you to help him make it back to the stage or if he gets grouped on. Beware of the grab though, if Wario gets grabbed, help him out immediately because it either means free damage or a stock.
Snake- This team is the best balanced team in Brawl (IMO). The team focuses on Partial Stage Control and a combination of Speed and Strength. Snake kills faster than you, can tank for you since he lives a lot more than everyone else but DDD with good DI, and he can break groups up easily with his explosives. One word: Omnigay. Snake's attacks have bigger hit boxes than yours do for the most part which can only be good as your range is almost equal to his and Marth's. The strategies of this team are obvious, meta takes control of the sky while snake is uncontested on the ground. Be aware of Snake's movements as he can get gimped pretty often and sometimes easily. The explosives also add an unnecessary (for you) layer of caution. You waste more attention trying not to get hit by Snake and helping him when he's off the stage at the cost of him being able to kill easily and to take damage like a sponge.
Diddy Kong- This team focuses on total Stage Control. You trade power, as Diddy Kong is somewhat weak, for total control of the ground. Obviously, Meta Knight takes over the air as the Diddy Kong rules with his bananas on the ground. Diddy Kong has various mix-ups so he isn't gimped easily. This is good for people who don't really have good chemistry and just feel like like playing doubles as if it were two 1 on 1 matches. In my opinion, this is one of the weaker teammates for Meta. This is because the heart and soul of Diddy Kong lies in the combos that follow trips from Banana Tosses so it isn't a good idea for the Meta Knight to interfere.
King Dedede-
I'll add more later. I got to go to school now
