I've played, although not finished all, every numbered title of TES series.
I wanted to write a personal assessment of each title I've played so far and compare with others'.
From the worst to best in order:
Arena
This game is very rudimentary. There are no side quests other than random quests composed of delivery and bounties. Lore is almost non-existent and even if you can get past quirky control and 2-d graphics, most maps are random-generated and have no merits to them. I played this game after Oblivion, and didn't like what I saw. I never finished the game.
Deggarfall
Too huge and lacks detail. It is a huge improvements from Arena, but what I saw was a feature-clustered mess. There are too many features that are either unrefined or broken balance wise. Dungeons were even worse than those from Arena. I played it after TES: Arena, but didn't like this one much better.
Morrowind
The first TES I played. Full with detail, and beautiful world to explore. Combat and dialogue were horrible, though. I didn't like the lack of fast travel and quest marker. I consider myself a semi-old gamer(loved BG1/2 and planescape tormet) but traveling on foot and aimlessly looking for a path/a small item dropped in a dungeon felt absolutely pointless and tedious. I like realistic experience, but those are just hindrances I could do without.
Oblivion
Massively improved on combat and dialogue. I really hated the leveling system and level-scaling. Bandits in glass gear and Marauders in daedric set, only 2 sets of which were found in Morrowind and its expansions, were just too much to take. Nonetheless, I would say it's an improvement overall not because of graphics but because of better combats and dialogue improved from keyword clicking.
Skryim
It has a feeling of morrowindified Oblviion, though it's been streamlined in some parts even in comparison to Oblivion. It's a compilation of advantages from previous TES without their flaws. The quests were still shallow and the narrative was not refined compared to other titles like TW2 or Mass Effect 3, but the Skyrim had the best combat, dialogue, quest verity in the entire series.