In my current lineup, I'd have to say skincare. I just switched over to Philosophy's line, and though it is more money than what I was previously using, I'm seeing a big difference. That's worth the money!
Second choice would have to be foundation, because I'm pretty tricky to match. But I was lucky I found a really great match without shelling out a lot.
Second choice would have to be foundation, because I'm pretty tricky to match. But I was lucky I found a really great match without shelling out a lot.