New Vegas.
I didn't know Morrowind is subject to quest-breaking NPC deaths (I have not played it yet). With Oblivion, I usually just walk carefully around most of the gates, except the ones I have to close. The dragons in Skyrim can get a bit tedious. I think in general, Skyrim is the buggiest of the lot though, which can definitely disrupt the relaxation value. Fallout 3 and NV rarely actually glitch on me, except in that they slowly break as my save file grows on PS3. But I believe that has more to do with the system I'm on than the games, and at least it is a predictable decline.