Im gonna say the Star Wars prequels. Both trilogies are a less successful offshoot of their predecessors but the Star Wars prequels for me fail on more fronts. The plot for them is far too dragged out and overly political in a non star wars way. The casting is all over the place. Whilst I liked the choices of Ian Mcdermit, Ewan Mcgregor, Christopher Les and Liam Neeson who all execute their roles perfectly, Hayden Christensen and Natalie Portman single handedly wrecked the movies for me with emotionally dead roles and a boring plot for them to boot. Artsitically I despise it because they didnt use sets in 2 and 3, they used sets in 1 which is why i like that one a bit more. CGI is meant to be a tool to aid creation of movies, not something you solely rely on and in 2/3 it was done over to death and it didnt impress me then and it looks even worse now. I feel also that the camera work was worse as well, the way they filmed shots just didnt feel right. The one thing I cant knock here is the music, John Williams is a credit to aucostics in general and him and the London Symphony Orchestra NEVER dissapoint.
The Hobbit on the other hand only failed in my eyes for a few reasons. Going against the lore, over use of CGI and a dragged out story(blame Jackson for making a third movie fromca short book). Because it was so spread out, the movie was padded with un necessary tid bits. When it got the lore right which was most of the movies, I loved it. The casting was top notch and although I criticise the overuse of CGI, it does wonders for the battle scenes, just amazing. If they had used more costumes for the orcs instead of CGI in the close ups, I would have loved it more(its why i appreciate the craft of Lord of the Rings). The Hobbit for me just felt like they wanted to take it further and up the ante but you just cant top LoTR. Star Wars prequels failed because they were an abrupt departure in every sense of the word and the real problem of it was the lack of emotion and groundedness. It was too dense and was all over the place storywise, all set up to get Anakin in that Vader suit whilst retconning a load of lore from the original trilogy. The travesty underpinning it all was that Lucas forgot what made Star Wars in effect Star Wars. Peter Jackson knew what made Tolkiens work a Tolkien piece but he was severely limited by time, 3 movies in 3 years, so I can respect Jackson for making what he made. It makes the Star Wars prequels more laughable as they were in planning since at least 1994 and each movie got 3 years production and look at the messes that were synthesised in that time span. So in my opinion, The Hobbit is just better