It depends on if customs are on and who your partner plays.
If you want a gimmick...
3xxx Palutena and Ness/Lucas/xxx3 Mii Gunner have a powerful gimmick where Palutena attacks with HL while the partner uses their absorption move.
Similar things can be done with Villager and Ness/Lucas through use of Pocket and PK Thunder.
G&W/Villager with any partner who has a powerful projectile make Oil Panic/Pocket a fear inducing tool (Vanish Bucket, MLG Ball, etc.)
Villager/Villager can perform the infamous rainforest/jungle technique by pocketing each others' trees.
Jigglypuff has always naturally been good in doubles with surprise/grab Rests and edgeguarding abilities to clean up.
Mario can FLUDD Rob's Uthrow off stage to get an instant kill.
Kirby can Inhale Shulk and become one of the best characters in the game by using his Monado Arts. Kirby with any partner who has a good Copy ability is useful, really.
Obviously there's a lot to list and I'm not even going to get into it.
If you want to avoid gimmicks, check out teams with really good synergy like Ryo and Sol, my favorite doubles team in Smash 4:
https://www.youtube.com/watch?v=e2VWSat9K_Y