Oh boy, I've been waiting for this one. Falinks is my favorite Gen 8 mon, so I was really saddened when I saw its underwhelming base stats. Luckily, it’s backed up by one of the best set-up moves in the entire game, No Retreat, alongside some pretty nice coverage. I've been surprised by how well it worked in SwSh UU (although I haven’t played since they banned Dynamax), and I’ve been using it in the Battle Tower. Here's the set I recommend:
Whoop Troop (Falinks) @ Sitrus Berry / Life Orb / Focus Sash maybe?
EVs: 252 Atk / 4 Def or SpD / 252 Spe
- No Retreat
- Close Combat
- Iron Head / Megahorn / Throat Chop / First Impression
- Rock Slide / Iron Head
No Retreat and Close Combat are necessities. For the third slot, One of the first three options I have listed are definitely better than First Impression, but if you’re looking for something a bit unorthodox, I’ve found FI to be quite fun (although probably just because I'm a sucker for quirky moveset options). Before setting up, First Impression can be used to get a surprise KO on a weakened Pokémon (since everyone expects No Retreat on the first turn you send Falinks out) before you switch out again, and after setting up, it can still be used if you Dynamax. Although if you’re playing a Smogon format, then First Impression is just going to be complete dead weight on Falinks.
Overall, I’d say that the best set is probably No Retreat / CC / Iron Head / Rock Slide.
EVs and nature are chosen to make you as fast and powerful as possible. I've seen some people run bulky Adamant, but you do actually get pretty fast after No Retreat, so I prefer capitalizing on that. For the ability, Defiant is far better than Battle Armor, to gain a boost from your opponent's Intimidate, Defog, Sticky Web, etc. There really isn't a best item for Falinks though, you can run pretty much whatever you want. I prefer Sitrus Berry to get some health back if you're hit while setting up, but Life Orb is also very good.