I just love all the crazy, lunatic ideas in BTTF2. Dehydrated pizza. Spinal treatments that involve tethering one's self to the ceiling and walk around. Flying cars. 3D shark movies with no glasses. Dust-repellent paper. Telecommunications systems with video chat and... fax machines. A paradox that may destroy all of existence or may merely be confined to our own galaxy. A dark timeline so horrific it's funny. And then the second half of the film where Marty time travels back into his own original movie and his past self ends up knocking him unconscious by accident.
There's also all the great, loving humour. Marty's 'inconspicuous' costume is a black hat and a leather jacket that makes him stand out in every scene. Doc Brown disguises himself as... himself. The hilarious hoverboard chase.
I find that most of my favourite movies and TV episodes are overstuffed with mad ideas never explored in great detail but used for striking and memorable visual effect and filled with incidental elements that make the landscape oddly plausible despite the plot's exaggerated absurdity and matched with comedic yet earnest characters whom you could lock in a closet together and still find interesting to watch.
I do think the first BTTF is a better film, but BTTF2 is the one I enjoy most.