Oh no...
Time has come for the face-off I didn't want to participate (the potential Bloodborne x Persona 5 will hurt too).
As much as it hurts, I'll vote for Hollow Knight. It doen't have the charm that Undertale has, but it is a superior game.
HK has tons of content, it is actually a steal for the price, could've been $40 and it would still seem a fair value for what it has to offer. It is challenging, has lots of variety in levels, story is nice, exploration is amazing and the soundtrack is outstanding. The game is hurt a little by the geometric level design, but it is alright.
Undertale is also amazing, but mostly due to its story, characters and uniqueness; the "gamey" part in Undertale is so-so, IMO. But nevertheless, it is also a masterpiece, no discussion about that.
Feels bad for Undertale getting out in the first round, but Hollow Knight is on another level.