I'm really not sure what I can rate as worst adventure game ever. I can give examples of most disappointing adventure games ever, though. BTTF is definitely one. Jurassic Park as well, though you can't really call it an adventure game. Still, it has remnants of being one, if only because the developers kept referring to it as the "betterment" of an adventure game. Saying things like "so we eliminated walking, because obviously the most annoying part of an adventure game is the long time it takes to walk back and forth between different screens." Oh yeah...OBVIOUSLY. Pro tip, if you're going to try to "progress" and "advance" adventures into the modern more accessible era then you don't go alienating the original fanbase by telling them why and how the games they've loved for years suck and need to be improved upon. That guy was pretentious as all get out. Oh well, at least the game did terribly.
I was disappointed in Escape from Monkey Island before I even got too far into it. It just bored me to death. Everything seemed like it was going so political in the story and it wasn't really about being a pirate anymore. I didn't care about what was happening and I didn't care about the characters anymore. I gladly left it on the shelf and never returned to it. Maybe I'll get back to it one day.
There are probably more but I can't think of any right now. What is the worst of these examples? Probably Jurassic Park....but it's not really an adventure, so BTTF, then. But do I think it was the worst adventure game ever? No, probably not. I'm happy to think that I haven't in all these years run into the worst adventure game ever yet.