I have a 29er single speed with a great frame and mediocre parts. I'm planning on going through the bike and upgrading components one by one but I'm not sure where to start since I can't fund a whole makeover at once.

One thing at a time, where will I get the most bang for my buck for performance? Wheels, BB, crank, brakes (currently disk but low end with poor stopping power) headset, stem, handlebars, seat post? Saddle is already done.