I have a lot of favorite video game series, so long post maybe? Final Fantasy, Mario, Sonic, Zelda, Star Ocean, Kingdom Hearts, Resident Evil.
Final Fantasy: My least favorite entry in the FF series would have to be Final Fantasy XIII. It suffers from linearity, a not so amazing combat system (imo; I played XII beforehand and THAT was a game ahead of its time...it felt like I was controlling my characters rather than selecting options from a menu), and a couple other problems like the inability to backtrack from CH1-10 iirc. XIII has amazing graphics (especially on PS3), amazing music, but beyond that I just didn't much care for it. Favorite characters: Lightning, Vanille and Sazh. <3
Mario: I have no 'least favorite' Mario game; they're all absolutely fantastic. Even though SMB3 is perhaps the hardest I've ever played, it's still...still on par with every other game made.
Sonic: 2006. There's enough said baha.
Zelda: Skyward Sword is my de facto least favorite Zelda title ever made at this point. Why? Honestly, I'm just going to say that the game is terrible. It's made essentially for a first playthrough and a first playthrough only; nothing you do afterwards is particularly mindblowing and there are few secrets to find because everything was made so that YOU WILL find it on the MAIN QUEST PATH, which is a bad thing in my opinion. I couldn't make a consistent list with the things SS does wrong, because it does so many things wrong. From picking some of the not so good things to reminisce about from past Zeldas, to faulty combat that gets boring after the first ten runs of that particular enemy, to music that is more "there" instead of being part of the game world, to extreme linearity a la FFXIII...gosh, the game just irks me. I'd erase it from history, but that's just me.
Star Ocean: SO4, The Last Hope, is my least favorite Star Ocean title. The combat got dull and boring, and there is an insane amount of grinding to be done if you want to get 100% Battle Trophies. It just wasn't as fun as Star Ocean Till the End of Time. The dialogue was cheesier than SO3's. The lip syncing was done terribly, haha. And the story felt a bit over the top. Graphics and music were amazing, but gameworld was a bit too huge so I spent majority of the time sprinting rather than playing the game. ;P
Kingdom Hearts: Truthfully don't have a least favorite title. KH games are magical in that they just don't disappoint; even if you have filler games like Coded or Dream Drop Distance, the gameplay ALWAYS makes up for the plot and overall you learn to love the games. If I must, though, my least favorite title is English Birth by Sleep, because Willa Holland and theguywhovoicesTerra don't do as good a job as their Japanese counterparts.
Resident Evil: Resident Evil 5 is my least favorite title, but this is odd because it's the title where I've poured the MOST hours into, thanks so much to Mercenaries and Mercenaries Reunion and of course the associated Leaderboards. See, RE5 is an oddity because I hate the lack of scare. I can replay levels all I like which is a good thing, but they're all set in daylight times for the most part and enemies telegraph their location which is ridiculous in my opinion. The music has a part to play, too, but it's really just because of the setup. Also, RE5 has the dumbest ever made character in RE history: Sheva. She has zero plot significance. She's literally there just to be there. Wesker focuses ONLY on Chris. Jill the same. Everyone focuses on Chris, Sheva is there just sitting around 99% of the time, and the 1% she is doing something is at the end of the game with rocket launcher OP amazingness. I love RE5, I really do, but it isn't on par with the rest of the series because too much action not enough scare.