If you want pk it's mostly flats, then 90s, then 60s, 33s is popular but very boring.
And the good thing about training more than one level type with this new rule allowing us to use more than 1 char in wild (without interacting) means you can PK in multiple places at the same time or wait for PK in multiple spots. For example you could PK / sit at the Territory Fort on Flats, the 13 wild Castle on 90s, in 5-6 edge wild on 60's and / or border for 33's. Nyx is definitely right, the popularity in brackets is pretty much in order to what he listed, 33's does go off sometimes and you see like 15-20 people there but I agree it's not much fun PKing in the same 2 squares for hours on end.
Good luck in whatever it is you're doing and hopefully I'll find you at one of those popular PK'ing spots and PK you soon. Cheers