Entry-level road components (tiagra)... If I had to choose ONE thing to upgrade, where would I see the most weight savings/performance?

1) Tires/wheels
2) Crankset/bb
3) Derailleurs/shifters
4) Fiber fork (currently aluminum, ~1lb)
5) Fiber bars

I'm really trying to avoid buying an entire groupset - which I am liable to do. I'm looking at the Ultegra/SRAM groupset.