If you're after the one sling that does everything, the Omni 360 is it. The new Cool Air Mesh variety has all the carry positions plus ultimate ventilation to keep you both cooler.
Omni stands for 'outward mode, no insert' and is the newest offering from the well known Ergobaby brand. It's main feature is that it offers a more ergonomic position for outwards facing than many carriers, by scooping the baby's legs into an 'M' position even when facing away from the parent (from approx 5 months). Unlike it's predecessor the 360 though, it doesn't require a bulky newborn insert for the first few months.
It also features the ability to cross the straps behind you (helpful for narrower frames and also not possible with the earlier 360 model), a lumbar support panel and a detachable storage bag for keys, wallets and phones.