Depends on the baby's age and size but...
newborn-6mo: Moby wrap or fleece kangaroo pouch (which I don't think this is sold anymore but it would be comparable to an adjustable hot sling I think?)
6mo+: Ergo, maya wrap ring sling
So far we've been using a moby wrap.
We're just about to invest in an ergo carrier since spring and hiking are coming soon!
lynn0926:My babies are almost 4 weeks old and we love the Baby K'tan. Very easy to use too.