Would be a tie:
1) I get the most use out of Savage Species; it has really helped me to open up the opposition that I can toss at my players without having to rely on a lot of guesswork; and
2) Alchemy and Herbalism; an oldie, true, but I picked it up a couple of months ago for a ranger player that wanted to get into herbalism more deeply than the infusion rules in MoTW, and it has led to lots of new roleplaying and adventure opportunities for him and the party. Never picked it up in the past because of being pricey (youch) but it was well spent.