To be honest, Swordfighter is probably the most versatile in all of his moveslots. Your preferred special moves will probably boil down to your personal playstyle.
Gale Strike is a reliable and somewhat potent projectile, though it has a bit of start-up lag before you throw it. Also, it has a windbox at the very end of the move, pushing away players it can't reach once it starts to fade out. Shuriken of Light comes out faster and fires faster, and it has the added bonus of dealing more damage the farther it travels. However, the projectile only has weak hitstun, making it only reliable for racking up damage (similar to Blaster on the space animals). I've heard you can gimp players trying to recover, but I haven't found it so useful in that regard. Blurring Blade is the most powerful of the three moves, though it has the drawback of leaving you committed to the move once you choose to execute it. It also has the obvious drawback of not being a projectile like the other two Neutral-Bs.
The Side-B moves are arguably even more versatile than the selections for Neutral-B. Slash Launcher is a simple forward dash that can be charged, and also moves forward when used in the air. Like the name implies, the attack launches foes upward if they're hit by the move. The user also plows straight through anything they run into, and can even dash through shields. I've heard it compared to Ike's Quick Draw (also a Side-B) though the "penetrating foes" property makes it unique in its own right.
Airborne Assault is ABSURDLY powerful, though it leaves the Swordfighter EXTREMELY committed to their move, and it also throws the user in Helpless state after the move concludes (on either a miss or a hit). If you barrel off the side of a stage, you can kiss your stock goodbye. Also, the Swordfighter bounces forward a bit even after hitting an enemy, which might cause them to careen off the stage if they smack someone sitting near a stage cliff (though you might be able to grab the ledge if you hold back on the Circle Pad).
Chakram is probably my favorite projectile on the Swordfighter, and for good reason. Using the move throws a circular blade a short distance in front of the user, after which it will stall in the air for about one second (unless it gets caught in stage terrain). Tilting the Circle Pad and pressing B throws a slow-moving disc that can hit multiple times, while using Chakram like a smash attack throws the disc faster and farther out, for a slightly stronger single-hit strike. Unlike Mega Man's Metal Blade, a Chakram stuck in the ground (or walls) can't be picked up by other fighters, but it also can't be picked up by the Swordfighter who threw it. Though it looks the same visually, Chakram's double duty is great for spacing opponents or keeping up the pressure, especially since you can tilt either input (slow tilt or quick smash) by holding up or down on the Circle Pad after inputting the move.
Your choices for Up-B are Stone Scabbard, Hero's Spin, or Skyward Slash Dash. Honestly, I find good utility in all three moves. Skyward Slash Dash can be aimed in any direction (even downwards!!) but has a slight start-up delay and the shortest vertical reach of all three recoveries. It also hits multiple times, with a slightly stronger "pop" at the end of the move. Stone Scabbard is the tallest recovery and also comes out the fastest of the three moves, though it has the obvious drawback of being a "suicide recovery". Contrary to popular belief, the move is not a clone of Aether or Final Cutter, as the Swordfighter attacks directly above or below their position instead of swinging in front of them like Kirby or Ike. The move actually functions closer to the Super Dedede Jump due to its hitboxes... though even that comparison isn't really fair, because Stone Scabbard can't be cancelled early, nor does it move as far horizontally. Hero's Spin is the sort of "in-between" choice, having a fair bit of height and distance. It also has two methods of attack, multi-hitting in the air but performing a single strong hit on the ground. It can also be charged on the ground, so it's basically a reskin of Link or Toon Link's Spin Attack.
Down-B is really the only simple choice. You can choose from Blade Counter (a counter similar to those of the Fire Emblem reps), Reversal Slash (which is almost identical to Mario's Cape), and Power Thrust (a sword variant of the legendary Falcon Kick). Personally, I always select the Down-B last, and use it to fill in the rest of the moveset. Is your Swordfighter one who loves to get in people's faces with Airborne Assault and Blurring Blade? Blade Counter's your best bet. Did you pick up Shuriken or Chakram, and prefer to mix up foes with a blend of melee strikes and projectiles? Give Reversal Slash a try. Do you like sitting on people but hate Stone Scabbard? Pick up Power Thrust.
Of note, Power Thrust is also a great choice if you pick the Chakram as your Side-B, since it's the one choice of the three that doesn't move your Swordfighter sideways (and Power Thrust attacks to the side while the user is grounded). Personally, I always run it alongside Chakram just so my Swordfighters have some method of gap-closer; otherwise, I find it to be superfluous compared to the other two Down-Bs.
tl;dr: Gale Strike, Slash Launcher, Stone Scabbard or Skyward Slash, and Blade Counter is the easiest to use moveset in Swordfighter's arsenal; however, nearly any combination of moves is effective, and boils down to your personal playstyle and preference.