For a newborn, a traditional ring sling or pouch is best. A lot of people like the Maya Wrap. It's unpadded, which makes it less bulky. If you have back, shoulder, or neck problems, or if you're small and have a big baby, you might prefer a lightly padded sling. Over the Shoulder Baby Holders are nice padded slings. Most people find heavily padded slings, like the Nojo, really hard to use. I like pouch slings, like the New Native. They're unpadded, and you have to buy a specific size, so they're a little less versitile. They're easy to use, but you will need a nursing shirt and maybe even a shawl or blanket to cover with if you intend to nurse in it.
As they get older, I like ones that center them on your body. It's not necessary for every mother-baby pair, but I'm 5'2" and built pretty small, and my babies are huge. My 14 month old has been in a 3T for several months. Some people can use ring slings or pouches as their babies get past the infant stage, but we just can't. My favorite carrier for older babies is the Sutemi. It's expensive, but it's wonderful. My little one is 28 pounds, and I can carry him for the better part of the day without any discomfort. I can even carry my 39 pound 2 year old for shorter stretches. I also like my Kozy carrier and Moby Wrap. The Moby Wrap is great for when you're getting the baby in and out of the carrier, like when you're running errands and getting in and out of the car a lot. You can tie it into place and leave it there, sliding the baby into it as necessary. It can get pretty warm, though.