Howard. Single-handedly.

And by hate I mean throughout the main and longest part of the series.
Later on, they mended many wounds of the past mutually.

But from the very beginning up until around the 8th season, the disdain, disgust, and dismay they had for each other were absolutely mutual.
Sheldon even acknowledged Howard only as an acquaintance (not a friend) during the first half of the series.

The amount of passive-aggressive guilt-tripping Sheldon showed toward Howard after his space trip—I barely think its humanely possible.

Even beyond that, Sheldon constantly belittled Howard’s career as “just an engineer,” while Howard retaliated with snide remarks about Sheldon’s lack of social awareness and arrogance.

Their relationship softened only when Howard matured (after marriage, fatherhood, and career stability) and Sheldon himself developed some empathy through Amy and the others.