Our loveable penguin has many set ups. As most should know, we are considered the best counter for snake. So i'll start with that. For approach issues, may i suggest that we start using (and abusing) a full-hopped Dair to approach and start new combos? For example, Full-hopped Dair leads to another Dair, a fastfalled jumped Fair/Bair, or a Uair. If you miss a combo attempt, grab and Bthrow. Of course, since snake's nades never suffer from SMN, we have to use waddles to stop the projectiles head on. And with the occasional Gordo and waddle doo, it can either just cancel or automatically blow up the nade. I'll stop there for Snake for now.
Next is Meta Knight. Meta Knight can abuse his B button and C-stick pretty easily. Which is why I suggest we start with double jumped dair (to lower the height of the attack and not have a laggy ending) as a approach and retreat with a Fair. From there we can at least attempt to throw a waddle/gordo and use dtilt to space ourselves. Since Meta Knight has an unstoppable recovery, I suggest actually using bair to cancel out shuttle loops and if all that fails, wait for the right opportunity to Utilt or Dsmash (although utilt is recommended because of its speed.)
I'll stop here because I havent exactly worked out a strategy for Olimar and Diddy