For me, Radovid.
1) it's most similar to how the books describe the future (obv not exact, plus it's different canon, but still) so makes most sense to me; 2) Given what he's just learned and witnessed in Witcher 2, plus everything else going on with Ciri that he's busy with, as well as the fact that all mages have already escaped to Kovir, I personally think it makes most logical sense for Geralt's witcher neutrality to prevail and thus for him to decline the assassination quest.
Obviously not portrayed as a good ending, although Nilfgaard ruling the North would arguably even worse overall. Dijkstra is my favourite ending but sadly the one that makes the least sense to be achieved, at least for me.
Once the witch hunts end a few years later, Radovid's reign is presumably stable but conservative and intolerant of dissent or secession. I suppose he'd be like a Russian Tsar or even Metternich-esque, which seems oddly fitting for the Witcher world imo.