it's tough but i think i would probably put my top 3 as:

pram - a good pram that also had a bassinet attachment. a lifesaver in a house without heating & a winter baby. it meant DS could sleep in whichever room we were in & we only had to heat the one room. it never even occurred to me before he was born that he would sleep in it for the first couple of months
bouncer - meant that i could have a shower without worrying that DS would wake up while i was in there. he also used to bounce himself off to sleep occasionally
books - pinky's books made me feel normal and they're something i've been able to gift on to other friends to help them feel 'normal' as well