TLDR: I didn't hate the majority of my experience in the game, probably would have given in a solid 4, but the ramped up difficulty of the Elite Four for no reason and my entire team getting swept by Aaron's Heracross brings my review down to pretty much nothing.
I was willing to not call this my least favorite pokémon game up until a few minutes ago when my entire team got swept by the first trainer in the elite four. Like many others, I wasn't totally sold on this game as it got closer to release for a few reasons, mostly because the art style wasn't up my alley and I was worried that the game just wasn't going to live up to the nostalgia of the original P/D/Pt trilogy for me. I waited until around Christmas to get the game and was pleasantly surprised by it despite the lack of Platinum content, the art style, the limited pokedex, and my overall terrible luck when digging in the grand underground. I liked the pace the game seemed to set and the changes to the escort missions that used to all be mandatory that I never cared for as a kid, as well as the following mechanic even if it does have its own issues that have already been addressed by the fan base at large. What really changed my opinion and cemented this game as my least favorite pokémon game is the outrageous difficulty spike between the final gym leader/final rival battle before the elite four and the elite four themselves. Don't get me wrong I think it's awesome that they gave the elite four members more competitive teams to raise the difficulty, but when your entire team gets swept by a singular Heracross there's a problem, especially when one of the pokémon that went down has no less than 3 super effective moves to counter it. The difficulty curve was just too outrageous and if this had been implemented correctly, we should have gotten the chance to select a difficulty at the beginning of the game. I'm a veteran player so this is only really a setback for me, but for any kids that are playing through their first pokémon game, this has a real chance to make them never want to play another pokémon game again.