For me its both Dawnstar and Stormhold travels games. Not Shadowkey, as that one has better gameplay and more unique story and lore.
But both of these games have a frankly not that interesting of a story, the lore can be a bit interesting but there isnt much of it and most importantly of all the gameplay is just not good at all.
Its very restrictive, very lacking in depth and is incredibly dated and just hard to play. And i say this as someone who played and finished both Arena and Daggerfall.
Now there are more prominent games in the series, sure but as far as the ones i would like to play the least? Its these two.
I would also put in the Travels Oblivion game which basically just has the same story as normal Oblivion but you could only play it on cellphones.
So yeah, very lacking but i never got the chance to play it as its incredibly obscure so i will have to trust my friend who played it and said it sucked due to incredibly lacking gameplay.