Every champion in the game can be stunned/parryed unless on certain nodes, or some hits cannot be parryed. Havok's mediums, Yondus Yaka arrows, Dominos Gun, IMIW's repulsers Nick Fury in phase 2, Iceman shrugs off when Ice Armour is active, any champs with Tenacity, kingpin Crossbones with fury's active. Any projectile attracts cannot be parryed, Nova and Korg also shrug off.
Emma frost is stun immune in her diamond form. Don't think their are any other who is stun immune
Other than that you might not be able to stun them at certain times because of non contact attacks, AAR or some other reason like everyone already said for domino, IMIW, crossbones, black widow
Thanks for some caracters to watch out for. I don't stun very good. I can stun at beginning of fight about 85% of time. I don't stun very much during the fight because I've practiced for over a year and it's just a waist of my health, I get koed. But I've noticed that some caracters I don't have a problem stuning but others I can't. If there harder to stun I can probably beat them better by interception. Maybe I just need to look for champs that are easy to stun.
Comments
Other than that you might not be able to stun them at certain times because of non contact attacks, AAR or some other reason like everyone already said for domino, IMIW, crossbones, black widow