If something is the schmaltziest, it is the most overly sentimental or cheesy.
The word schmaltziest is usually used to talk about things that are too emotional or too sweet.
Examples:
That movie is the schmaltziest one I've ever seen.
The love song is the schmaltziest song I know.
The schmaltziest part of the story is when the puppy finds its way home.
The cake was so sweet, it was the schmaltziest cake I've ever tasted.
Her room is the schmaltziest room in the whole house.
The movie was the schmaltziest thing I've ever seen, with so much crying and kissing!