I've been playing Swain Top a lot recently and I haven't really found any champions that gives him difficulties other than Darkin Kayn (which is mainly played jungle).
I understood that Drain Tank champions are the ones that counters him but so far I've never had any difficult matchups.
Slythrin Umbreon3 votesSwain's just a little bit overtoned at the moment. He was strong on release and then they just kept buffing him... Never Learn, Never Change, Never Move.
Hextech OwI1 votesThey just nerfed him last patch...
Nut on my Butt1 votesSustained melee attackers that can really stick, like Irelia or Fiora. The closer you are to him, the harder it is for him to snare you. In Fiora's case, if you're in his face, his snare becomes extremely telegraphed and it's basically a free parry stun for you.
saltran1 votesAnother Swain /s
OurLestrade1 votesAll about divers with good combos, especially early game where he's an immobile squishy mage.
Ahristocats1 votesROFL
he has phase rush to get out, and he fcking isn't squishy friend
The Minion hunt1 votescammile
Dokkaebi1 votes
Ahristocats1 votesRumble and that's it
Meep Man1 votesI like 
cause of his 30% MR increase
is also decent, but kind of meh
Swain is a huge ball of damage and his root makes champions that would normally be able to punish Mages perform more poorly against him. Not to mention his Q and arguably W punish you for diving on him hard. It's best to just pick someone who can take a hit and then also collapse on him when you get a gank.
Håppy1 votesIrelia was giving him the ol' nae-nae, but after her hotfix, I don't think she'll be out-fighting anything for a while
Salamencę1 votesDaddy Darius does the job quite well. He uses Swain's pull against him xd.
Rico Nasty1 votesTank 
Top negates his existence.