Who did benefit most from the north stream sabotage? Not Russia, nor Ukraine, but the USA. Their gas replaced Russian gas imports.

The sabotage happened after a simple political decision had been made to turn it off (more accurately to "suspend its certification", as it had yet to actually enter service). So there was never any "need" for sabotage. In any case the sabotage as such had no effect on gas imports.

Who did benefit most from the north stream sabotage?

At the end of the day -- nobody of course, as the whole idea that the sabotage could bring any significant strategic benefit (even in terms of the "psychological" front) was pretty much braindead from the start. Meanwhile all it seems to have brought to the table was added instability, more paranoid thinking all around (in addition to the quite substantial methane release).

But it's definitely easy to see why (at least some of) the Ukrainians thought their side could gain something from of it; or they may not have been thinking in terms of any specific strategic advantage, but rather simply spite.

Either way -- the decision was made, and the job was done.