Here's a list I got on Surrender@20. Not sure if all interactions are included, but this should be most of them:
Items:
"These shoes... they are too tight!" (Boots of Speed)
"No sneaking up on Braum this time." (Sightstone)
"Sight is the key to victory... also stone." (Sightstone)
"Fire does the body good." (Sunfire Cape)
"I've always wanted an on-fire cloak." (Sunfire Cape)
"Ooh, now I need marshmallow." (Sunfire Cape)
"To friends I am snowfall, to enemies I am avalanche!" (Frozen Heart)
"Ha-ha! Perfect!" (Boots of Mobility)
"My shield will protect this shield." (Randuin's Omen)
"A shield for my shield? I like it!" (Randuin's Omen)
"This has the smell of a faraway land. Perhaps I shall see it one day." (Ancient Coin)
Poro Snax:
"Eat, eat and grow strong!"
"Train hard little poro and one day you will be strong like.. baby."
Alistar:
"You remind me of Agatha. Best cow back home!"
"Alistar, I have cow you should meet."
Annie:
"Mother gave me bear as child too, Annie."
Graves:
"After this we share a drink. You like goats milk?"
Gragas:
"What's your poison, Gragas? Mine's milk."
Tryndamere:
"Tryndamere! This is fun, right?"
"Are you having fun, my friend?"
Vi:
"Your hair... we do not see things like this in the Freljord."
It also looks like he has some special interactions when blocking certain abilities with his E. I couldn't get them to trigger in game but here are my best guesses based on the files:
"Look, your axes bounce right off" - Draven Ult
"Good Shot, you'll get them next time" - Ezreal Ult and Caitlyn Ult ( ? )
"I'm having cold flashes" - Ashe Ult
"Heh, it is windy today" - Riven Ult
"Ah! A refreshing sea breeze" - Nami Ult
