Toxicroak is definitely the best counter because Suicune can't do anything to it, and unlike other water immunities which Suicune can still attempt to PP stall, Toxicroak can just set up to +6 and slap it without a care.
Articuno is another favored counter of mine, I've actually tried to play Suicune into Articuno before on the Mega Sableye ladder (Where Articuno was everywhere) And it definitely absolutely hard stops it everytime.
Favorite check (Which honestly feels like a borderline counter) is Hydreigon. It's my favorite check to splash onto teams and taunt + water resist + recovery definitely stops Cune dead..