I think Justice For All takes the cake for me. Uninteresting cases, some boring designs/characters, uninspired music (for the most part), cheap wins (Phoenix pressures the true criminal into submission for almost every case through mostly conjecture instead of, say, definitive evidence or powerful bluffs [like what he did in case 4 of the first game] ) and the magatama sequences were too dragged out. Of course, I can say the same for Trials and Tribulations (the magatama part) but at least that game had some really great things to make up for it.
Though case 4 was pretty great (what with having a reformed Edgeworth returning in full glory and being a decent case overall) and all in all, it wasn't a bad game, just a very poor sequel to the mind blowing first game.