For me it is Enterprise - and for similar reasons as Discovery: Doing good prequels is no easy feat. You will always have to tangle with canon history, the acceptance (or lack thereof) in regards to updated visuals that might clash with prior works and the temptation of using established characters and concepts that simply shouldn't be tied to the new work for various reasons.
Back when Enterprise started I still had hope that they would use the 'prequel'-situation for some serious storytelling in terms of giving us a front seat for the whole 'birth of the federation' thing. However, I quickly felt cheated by the elements of that which actually made it into the show. A lot of what happened didn't necessarily require for Enterprise to be a prequel - and in many regards they ignored or quickly abandoned interesting concepts and challenges. The transporter, that the captain initially wouldn't use on his dog was pretty much the go-to option about five episodes in. Warp Engines and Weapons felt very 'samey' to their modern counterparts, just with a lower version number. The grappler as an alternative to the tractor beam was perhaps the most iconic difference and to me it still felt underused.
Star Trek is to me a vision of a better tomorrow. Something we aspire to achieve. Which is why I want new journeys and adventures to play out in the future, so that the story goes on and we can push some more boundaries, challenge what we considered to be the limit. I approve of exploring past events with flashback episodes every now and then, but as a whole I don't want to go backwards, I want to boldly move forward.