Tomb Raider (we all know why) and MGSV in my opinion both of them seem to be doing not a little worse then I personally expected of them but allot worse. December showed some recovery for Tomb Raider but I'm not sure how much it will do in the bigger picture of things, it still looks very bad to me. MGSV being multi-platform makes it a pretty big surprise that it's not selling better then it is, given what MGS4 did makes it a big disappointment.
Over all, I'm undecided on which I'd put over the other as worse yet... Tomb raider has 2016 releases but if I do look at just 2015 Tomb Raider takes it for most disappointing sales as it's sales on Xbox have been extremely underwhelming and half of what you would expect from the franchise (again, we know why). Which is why I'm not throwing Halo 5 in there, it's in a pretty big decline but not as bad as these two in my opinion for most disappointing sales.