Morrowind is a game I loved (probably one of my top 3 games of all time) but which I can not return to now. It is not an issue of graphics, but the combat system and the static NPC's that make it hard if not impossible for me to return to. In its heyday it was an amazing game.
As for the other games:
Oblivion I had a "Phantom Menace" or "Crystal Skull" moment in that I wanted to like it so badly but I really didn't. Sure, it has those two elements that I mentioned above as improvements from Morrowind, but beyond that it felt lacking. Mods changed that a bit, but I still feel Oblivion is Bethesda's weakest RPG yet.
Fallout 3 I ignored for quite some time because it wasn't a TES game and I was probably a little spiteful about that. Then one day, after watching the Mad Max movies again, I read about it and figured I would give it a try. I enjoyed it quite a bit and it returned my confidence in Bethesda after my distaste for Oblivion (in Vanilla form). Some things were odd and felt off (the willy-nilly placement of creature types and raiders around the wasteland with no logic) but I still like the game.
Fallout New Vegas I enjoyed from the start but became a little disappointed in only because it didn't feel wild enough for my tastes. That being said I still like the game a fair amount and the world seems more logical in its organization...well, as logical as a Fallout game can be. Really wished the desert expanses were larger though...a thirst meter is useless when fresh water is within sight of any location in the game.
Skyrim is probably my favorite (roughly equal to Morrowind) because it was better than the (my opinion) flop of Oblivion and its Nordic/Germanic feel sit well with me (I almost always play as a Nord) having been a huge fan of Bloodmoon. Although I was disappointed with the lack of deep wilderness one would think exists in Skyrim and the rather small cities, I love the game otherwise. The only further distaste I had with time was the lack of terror dragons inspired. When I began chasing them like monarch butterflies it was obvious they were not what they should be,