I have a special nostalgia with Oblivion, so of course that is going to be my vote. Oblivion was the first TES game I ever played and is what got me through a particularly rough and isolated time of my life. I wish I had the same experience with Morrowind when it was released in May 2002, but I wasn't really into the fantasy genre back then.
For new players of the TES franchise I would recommend the latest (which, at the time of this writing, is TES V: Skyrim). If they get hooked on the lore and storyline then they can look into the other TES games. But generally for newer players in any franchise, it's best not to start out with older generation graphics because it will be an unfamiliar/degraded experience which won't leave the best lasting impression.
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Autistic (self-identified)
Open source, free software, and open knowledge geek
GoLang, Python, & SysAdmin aspirant
RPG enthusiast
Has OCD, social anxiety, CPTSD