Sierra certainly didn’t’t have a monopoly on awful games; they just had a lot of the more expensively produced ones. Here you can get a little taste of ten horrific games I sat all the way through so you don’t have to. Caveat: There are a lot of games that are far worse than many here; they were just so bad I never bothered to play them for long.
10. Star Trek: The Next Generation: A Final Unity
9. The Lurking Horror
8. Runaway: A Road Adventure
7. Cruise For A Corpse
6. Syberia
5. Silent Steel
4. Leisure Suit Larry 1: In The Land of the Lounge Lizards (SCI Remake)
3. Monty Python & The Quest for the Holy Grail
2. Law & Order: Criminal Intent
1. Circle of Blood
Death By Troggles 
I’m kind of disappointed that nobody caught this, but my #4 game on the list was made by Sierra. I’m going to leave it there for fun.