The rolling parts of the bike. Conventional wisdom is that saving weight here has much greater value than on the rest of the bike, which is true, as you have to provide two types of movement: the forward momentum of the bike as a whole, and the angular momentum of the turning wheels. Add in the impact resilience provided by the spokes and the aero effects - resistance to forward velocity and the vulnerability to cross winds - and wheel choice is crucial.
At Hubjub we are all about the handbuilt wheel made from metal components and by way of example we have included a complete wheelset here. We build to order and have a growing reputation for value. Generally we use products you see on the site but can usually source others as you prefer.