I'd slot Walls as the #1 priority.. I find the extra slots helpful.

Rally Point would come in at #2.. more troops per wave = love.

After those, I'd consider Feast Hall or TH.. Town Hall for the extra valleys/resource slots, Feast Hall for the extra hero so you can use that lev 10 rally spot to its fullest. Really depends on your preferred style of play though... a more aggressive player might hit Rally as #1 with Feast as #2.