So I think there is a case to be made for Monero here, although objectively I'm not sure it will have "the biggest impact". In fact the biggest impact may be negative, and I can think of some high profile efforts that might turn out to be colossal failures in 2015.
I think the technology has already had the theoretical, psychological impact on the space in 2014. In order for Monero to really have an impact in the "cryptocurrency landscape" in 2015, one of the following would have to happen:
a) Its technology is implemented in bitcoin (not going to happen)
b) Its technology is implemented in a sidechain (more likely, but sidechains probably won't exist in 2015. too early for that)
c) It becomes the dominant currency in certain niche markets (too early for that)
2015 will be competitive in terms of impact. EtherEx on Ethereum could rapidly make Cryptsy and other crypto-crypto exchanges obsolete. Overstock's Medici on Counterparty would be pretty big. And as you said, some sort of colossal failure might easily end up being the most impactful thing. It's truly hard to imagine XMR being the #1 crypto story of 2015.