I don't know exactly what you mean by "RPG Mechanics", but Morrowind overall holds out the best:
1) scaling of pretty much every derived stat, attribute and skill beyond 100 allows for the most consistent potential for power level increasing - even combat skills above 100 can have their purpose for increasing chances to hit against enemies with sanctuary/extremely high agility/luck, similarly magic schools, willpower and intelligence can allow for better spells that your character will be able to cast and have the spell points to cast them. In Daggerfall and Oblivion, skills and statistics are either overpowered due to bad coding or functionally indifferent above 100 (all spells costing minimum of 5 spell points in Daggerfall if you go above 100 with enchanted items, in Oblivion skills don't do anything above 100 and attributes don't work for derived combat stats above 100).
2) this deriving further to magic items - Morrowind has a plethora of very powerful magic items to use, both constant effect and on-use enchantment that can vastly improve the power level of your character. Daggerfall premade artifacts are highly disappointing for the most part, and even though custom-made magic items can be very powerful, the limitation or buggyness of stats as aforementioned takes away from the fun of it. Custom weapon enchantment at least remains a solid feature in Daggerfall, although Morrowind's magic effect combination generally works better in my experience. Oblivion's custom enchantments are for the most part highly disappointing, and without mods, most artifacts' usefulness is limited by their level-scaling actively discouraging you from getting them at early levels.
3) further on enchantments, they seem to have got the best balance on their functionality in Morrowind: with the enchanting skill affecting the cost of casting, it DOES get pretty abusive potentially, reaching similar levels of Daggerfall's spell cost being minimalized - probably an expectable trade-off considering that while spell costs decreased with skill levels in Daggerfall and enchantment cast costs decreased in Morrowind, this would happen sooner or later - be assured though, I am as critical that this can happen in Morrowind as I am for Daggerfall, albeit I find it a bit less overpowered since making custom enchantments is more effort than spellmaking, and reaching the 113 enchanting skill also takes considerable effort - plus maximum power of individual enchantments is limited by item capacity, something spells are not limited by as easily. Regardless, with the way the costs work and how well you can customize the custom spell effects for enchanted items as well in Morrowind (in Daggerfall enchantments only allow you to use combinations of premade spells and not full spellcrafting), it still wins out. There are many enchanters in the game selling custom items with exactly those kinds of various effects as well for exactly this purpose. Oblivion disappoints, as its only active cast enchanted items are magic staves, and you cannot even customly enchant those.
4) exploration skills and spells were amazingly done in Morrowind, even though not strictly necessary, however the Jump and Levitate spells were both working much better than in Daggerfall, and allowed for great exploration potential in both dungeons and outside world. Daggerfall lacks exploration of the outside world, and its levitation was pretty wonk. The climbing/swimming skills were ultimately made unnecessary by spellcasting, and thus rightly removed from Morrowind and further, although admittedly the climbing was fun conceptually. Oblivion, of course, having its nonsensical dungeon design with slightly-bigger pits and slightly-too-high-to-jump passageways had to remove exploration spells or else you'd be able to break the game too easily. Disappointing.
There are some things that are better in individual games and worse in others, and in the end the systems are pretty flawed, I just find that Morrowind worked out best on average (although it is not noob-friendly, stamina is very punishing in the game and without realizing that you should optimally buy some cheap magic items to restore it without usage of potions, you will struggle through the early game massively or waste a lot of time just walking instead of running).
You may notice I pretty much only made comparisons between Daggerfall, Morrowind and Oblivion, and that is because those games in the end have the most directly comparable systems. Arena and Skyrim both diverge further from that "TES basis" and thus I didn't find them directly comparable. To give some footnotes on why they are not my favoured: Arena is just really flawed, bad itemization, overpowered magic, strict class based gameplay, little customization. Skyrim showed some potential in enchanting in terms of constant effect, but disappointed with premade magic items and artifacts to make up for it. The perk system in Skyrim is an improvement from Oblivion (although not without its flaws which I will not divulge upon further here), and Oblivion's skill-perks was already a nice feature compared to Morrowind, something I will give recognition to. However where Skyrim really falls is the magic, which lacking customization ends up being very generic, its "dual wield spells" concept really doesn't work out in any interesting manner in the end, and is a major downgrade compared to Oblivion, where custom spell chaining and combinations that were easy to cast during combat were by far the best part of that game.