Honestly, I love Sylveon because of its bulk. This is one of my favorite sets to run, and while Sylveon's speed can be its downfall. I feel like this set could work really well with different EV combos, too, depeding on your own play style.
Sylveon (F) @ Big Root
EVs: 252 HP, 126 Def, 4 SpDef, 126 Spd OR 252 SpAtk, 252 Spd depending on play style
Nature: Modest (+ SpAtk -Atk)
- Hyper Voice
- Draining Kiss (restores 75% of damage done + an extra 30% from holding Big Root)
- Calm Mind
Hyper Voice should come standard with Sylveon. You can bypass other Sub users, which is a must because it's likely they're going to use that to set up (just like with this set). Draining Kiss is going to be your key recovery move here. After a few Calm Minds, you're going to regain a LOT of health to keep fueling your Substitutes. Calm Mind is self explanatory. Boost your SpAtk and SpDef to soak up more special attacks. Substitute makes for great stalling and a good set up buffer--just be wary of your standard speed status leads, like Furfrou.
Obviously not prioritizing Def makes you weaker to physical hitters, so it's up to you to decide if you want to try and outspeed (good luck) or outlast. When I first got this girl, she had the second set of EVs. She was decent, but couldn't take a hit from Fighting types. After retraining her, she's a hell of a wall.
If you've got any tips on how to improve this set, comment and let me know! This is my first real set, so I'd love to learn more.