My favourite flats on a P are TI's, I believe they are what Pino uses also. Lots of midrange character, very compliant, and silky smooth once broken in.
I also really dug the Pyramid Golds for a bit more heavy thud, they also sound broken in straight away and are the smoothest feeling strings ever.
I didn't really get on with Chromes or LaBellas - too high in tension and they felt stickly compared to the TIs and Pyramids.
The last P I bought came strung with Rotosound flats and I was very impressed with those too, very bright sounding and nice to play, probably a fairly new set. I took them off in favour of rounds fairly quickly as I already have another P with TIs, but I think I will put them back on at some point and give them more playing time.