I grew up during the late 80s early 90s and own most of the Sierra and Lucasarts adventures. This question is a no-brainer for me – I love playing Sierra games far more than the Lucasarts ones.
This is only my opinion, so don’t get too offended by it. I’d rather have a constructive discussion than an argument. So take it with a grain of salt.
There are a few points behind my thinking.
1. Sierra games had far superior stories – this the key point for me. For the most, their games had great story lines and fantastical worlds to adventure in. I found the Lucasarts adventures to be more focused around humour and sillyness. Stuff like Gabriel Knight, Manhunter, Police Quest and Quest for Glory was brilliant for me.
2. The themes of the Sierra adventures covered such a wide range of topics. Police, Space, Manhunter, Heros, little kids, historical. There were so many variations that I never got bored, and they catered for all kinds of gamers. I used to love playing Mixed Up Mother Goose and Black Cauldron with my little sister as much as I loved playing Police Quest and Quest for Glory with my brother.
3. Sierra was far more prolific – there’d always be something new to play.
4. Finding funny ways for your characters to die, and then watching the cool animation and laughing at the sarcastic comments.
5. The characters in the Sierra games were just so lovable and memorable. Their is a big emotional investment there too. I loved seeing what adventures Roger Wilco, Leisure Suit Larry, Gabriel Knight and Sonny Bond got up to. I never really cared for the LucasArts characters as much. GuyBrush was cool, but he was basically just an imitation of Roger Wilco and Leisure Suit Larry anyway.
I do admit that LucasArts had a higher quality ratio, but the argument about Sierra dying slowly and horribly is exactly the same (to me) as what happened to Monkey Island after Ron Gilbert (I think) left. It basically became a lame parody of itself. And some of the Sierra series finished off really well (for me) – I loved the last Space Quest, Gabriel Knight and Larry games (the new one doesn’t count IMO – its not even Larry in the game).
Also, I think Monkey Island 1 is one of the best ever games I have played, and probably as good as any of the top-drawer Sierra games (of which there were only a handful IMO).