I was a bit confused on the question at first, but after careful deliberation I said during BatP. I was going to go with PML but I figured there's a lot of stuff out there dealing with PML although wouldn't that be an interesting twist (definitely would have put that as my second choice).
But BatP happens so late in the season, plus it would come with a whole slew of other issues, what with the Planet problems and Lois' engagement and Clark's confession of love... And then something happens where it all hits the fan and they find out they're soul mates, in whichever manner you please. It would be wonderfully chaotic.
But PML definitely for second place. For very similar reasons. Also, if it's right after PML, then you'd have the added consequence of Lex having just fallen for Lois, and Lois now knowing she's destined to be with Clark... Lots to go on there too.
Any way you choose, I am excited to read it!