I think it's a gimmick but classically people partner Ness with someone with an energy projectile.... since Ness can heal from that projectile using his DownB, PSI Magnet.
If I were to rank some of these characters, based on a combination of the character's synergy and all-around goodness:
Lower
VVV
Robin (nah)
Samus (charge shot is a bit fast and scary to react to, even though it heals a lot.)
Lucas (PK Freeze is slow, SideB doesn't fully activate on PSI magnet and thus doesn't heal much, his PK Thunder is slow. On the plus side, he can heal from you)
Mario (he only has fireball, but it's easy to see coming and heals a fair bit. Plus, Mario has classically been a well rounded character for doubles)
Mewtwo (shadow ball is YUMMY but a little fast/scary, like Samus' charge shot)
Pikachu (thunderjolt / thunder are easy to see coming, which is good for you. If Pikachu's thunder connects with himself though, you can't heal from that! It'll hit you full blast. You have to jump into the lightning as it's coming down.)
Pichu (see Pikachu... his electric moves, while they heal you a lot, also hurt your partner a bit. Maybe not so good.)
Ridley (get off a stream of fireballs and you can heal SO much %. The normal heal cap is irrelevant since he launches 6 separate fireballs.)
Isabelle
Villager (he can pocket SideB and us it on opponents for big damage, or pocket your upB and toss it back for huge healing, because his pocket makes the move stronger and thus heal for more. The sequence of, Ness uses upB -> villager pockets -> villager throws it back -> Ness heals is fast and easy, because of the huge range on pocket and PSI Magnet. The two also just have great synergy, with Ness scoring easy kills with backthrow and villager repelling enemies with slingshot. By far the best pick, and since I think villager is better than Isabelle, if there's no preference, go with Villager.)
^^^
Higher