ERGO! No question! It holds kids up to 35 pounds (probably more than you'll ever want to carry), plus they SLEEP so well on your back! And it folds up VERY small, unlike the big "hiking gear" ones, so it's easy to throw in the diaper bag or car, or even a large purse.
My kids slept so well in them, it got to the point where I ALWAYS went to the bathroom just before putting them in the Ergo carrier, or I would end up having to wake them up so that I could use the bathroom.
They have to be old enough to hold their heads up, so at least 5 months old. And you can carry them on your back or front. The back is easiest, because it allows you to do more, and on the front they have to be facing in, never out. But that's not really an inconvenience, 'cause when they face foward like the Bjorn carrier, sooner or later you're gonna get kicked! (Although that's a LOT more painful for dads.)
It may take some practice getting them in it when you're alone - it's kind of awkward - but it's totally worth it because it's very comfortable.
(Although I should second the person who said for SERIOUS hiking, get something with a frame. Ergo is more all-purpose, good for long days at the zoo, festivals, shopping, stuff like that.)