Depending on the situation: Awful Boring it's a disaster It's a waste of time. 'Sucks' can cover a wide range of negative adjectives. Take your pick

Other slang words you can use are "blows" or "stinks"

Sucks can mean almost anything. And hate to be a grammar nazi, but "Which word can I use instead of the slang word 'sucks'?" sounds unnatural. I would type, "What words could I use besides the slang word 'sucks ' ?"

I like to use the word that "bites" instead of "sucks"

that sucks= that is disappointing, terrible etc