RockShox Reverb, but it was hard to set up - once it got dialed in, it's been great. I've heard good things about even the cheap dropper posts, but it's important to get the right size for the bike you have.
Another game changer is flat pedals/shoes. If you are riding technical stuff, or just want to be able to walk around,
I highly suggest getting these shoes and pedals. I had one too many low speed crashes in my clip in pedals in the backcountry and was willing to sacrifice uphill performance - turns out, I sacrificed nothing and ride better knowing I can put my foot down whenever I want.