I don’t have enough time to play enough games to really give you both a best and a worst. So instead I’ll mix it up a little.
2015 Best Game of the Year: Undertale
Ha! I know this may be met with some disbelief, or generally confusion (what’s Undertale?) It’s a mostly solo project that you can play through within 9 hours, and that’s if you’re taking your time. And it looks like an extraordinarily cheap JRPG clone. The story isn’t ridiculously bombastic or all encompassing or expansive, either. But this is a standout game because it was so thoroughly and lovingly crafted. There is not a single detail that has been left untended to, and every action you do take in this game has a consequence on a whole new level unlike any other. And the whole premise, when you look at it more closely, actually really gets down to asking you how good you are, as a person. If you let it, it can be a very personal experience. How humorous, chilling, tragic or uplifting the story ends up is up to you.
NB: does require a fair degree of familiarity with ‘internet humour’ to fully appreciate.
If you liked Undertale you may also have played: Lisa: The Joyful
This is technically an expansion, or a sequel to the larger game, Lisa: The Painful, which was actually released in 2014. It also has a JRPG like interface but achieves its profound emotional impact in an entirely different manner, most easily described as “a thoroughly fucked up post-apocalyptic story”. Warning: has strong elements of familial abuse. The Joyful, in its own way, is the twisted reaffirmation of the determination and the will to live of the character key to the protagonist of The Painful, as well as the answers to the multitudes of tortured questions the player is no doubt asking themselves throughout The Painful.