I always just sort the servers by population and go to whichever siege or battle server has the highest population and play there. One of my favorite things about this mod is that as long as you can block up and block down, you can still be competitive even with 180-200+ ping.
I think some of the weekday population drop might have to do with the small number of maps in rotation on the two most popular siege servers, it gets kind of boring switching back and forth between the same 3 maps. Also battle maps can turn into long drawn out campfests, I'd like to see more open field style battle maps so the action moves along quicker.
Probably the most fun I've had in this game was on the NEC (east coast US server?) the weekend after the game came out. An admin was having each side take turns charging at the 5:30 mark. It kept the game moving along and was really fun to fend off those human wave attacks or go down in a blaze of glory on the attackers side. If there was more hands on refereeing like that I'd play on whichever server it was happening on, even if the population is lower.