Kirby vs Samus (SSBM)
Mention this match-up to any Kirby main (all six of them, lol j/k) and you'll pretty much get the same jaded response (*groan*). Samus has been voted as Kirby's biggest counter by the majority of Kirby players out there, and for good reasons.
First of all,
any character who has a projectile automatically has an advantage on Kirby. This being because the only pseudo-projectile Kirby always has access to is his Final Cutter, which does diddly-squat when facing Short-Hopped Lasers, Needle Storms, or even Luigi Fireballs. If Kirby attempts to use Final Cutter (Up+B) as a projectile, it has a terrible start-up; a predictable path; and, perhaps the worst part, if Kirby gets hit he'll lose all his jumps.
Now, without a reliable projectile to hide behind, advance with, or otherwise spam, Kirby pretty much is forced to move in close. And looking at all the long-range options Samus has, missile-cancelling, two varieties of missiles, and even morphball bombs, it's a wonder that Kirby can get close at all. Since he's slow, he has to rely on his ability to duck under projectiles, jump, or otherwise dodge until he can make his way close enough to become a threat.
You might think that since Kirby is lacking in the long-ranged department that he would be a great close-quarters fighter, or at least have some tricks or advantages while up close to help out. Well, forget that; Samus has far more options and better attacks than Kirby could ever compete with. The bane of Kirby is simply Samus' power in relation to Kirby's light weight and easy-to-combo marshmellow body. Practically anything Samus hits Kirby with will send him off the stage, launch him into an aerial, or out-right KO him. Combine this with Samus' outrageous crouch-cancel (usually into d-smash), and you've got yourself a bona fide Kirby Counter.
So how is Kirby suppose to fight against Samus? Well, there are a few things Kirby's got up his non-existant sleeves. For one, he can be hard to hit with missiles while ducking - though this is only an on-the-spot remedy since good Samus players can send smash-missiles along the ground close enough to hit Kirby and not-so-good Samus players can just switch to homing-missiles. This can, however, open up opportunies to move within striking range of a missile-blasting Samus.
From there, you need to know thy enemy. If you expect your opponent to go offensive, prepare yourself for your counter attack. If you expect your opponent to go defensive, move in as appropriate with feints, draws, or the proper attack at the proper time. For instance, if you expect Samus to slide back, you could dash-dance to give the allure that you're moving in for an attack, and then back out of harms way (if you expect a F-smash). If you're facing a crouch-canceller, dash in for a grab (Short-Hopping into D-air can also work if you've got L-cancelling down pat).
Kirby has a few more saving graces that keep him from being completely helpless at the hands of Samus. For one, if you can get a Copy off on Samus, Kirby will then have access to a useful KOing move. Samus' charge beam will give him that much-needed long-ranged attack with the addition that when fully charged it can cut its way through missiles and still hit with enough oomph to KO. Also, Samus is sort of slow in the air, which at least gives Kirby time to go offensive. The Up-tilt combos into B-air and U-air nicely on Samus which can get her off the stage where she has to focus on recover rather than firing off missiles. In fact, KOing Samus is so much easier when she's off the stage because one Kirby d-air can spike her far enough down that even a Meteor Cancel won't get her back up.
So, even though it's debatable whether or not Samus is Kirby's Number One counter, it's guarenteed that Samus has enough advantages to be high on his list.