Like pps have said, it depends on your needs. You asked what type of sling was best but I assume you meant baby carrier in generaly. Before looking at the different brands, you would have to explore the options of soft backpack carriers (like Bjorn, Ergo), frame backpack carriers (like for trail hiking), slings, shells, or wraps.
When it comes to backpack carriers, most people have a soft carrier. The Ergo while more expensive than the Bjorn puts your babies hips and legs in a healthier and more comfortable position. Also, the Bjorn you have to gradually get new sizes because they weigh out. The Ergo (with an insert), you can use from birth until ~40 lbs. The only downside to the Ergo is that the carries are only face-in and it can be difficult to nurse in one if you are large-chested.
When it comes to slings, they are great for cradle/football holds, kangaroo holds face-in/face-out, front hip, back hip, back. A ring sling is also wonderful for nursing. I would recommend the Maya wrap or something similar. You can also make your own very inexpensively and generally in around 30 minutes. You can do similar holds with a shell but there isn't a lot of room for adjustment, but that can be very good for some beginners who are nervous about baby wearing.
When it comes to a tie wrap, there are as many different ways to carry your baby as you can think of. Most moms work with an immediate repitoire of around 5 different carries at a time, but those change with your needs and your child's size. A tie wrap is certainly the most advanced way of carrying a baby, but you can address any physical needs that you would have directly (upper back pain, lower back pain, hip problems). You can also leave a tie wrap on and just take the baby in and out of it without needing to re-tie it every time. Just be careful of what kind of fabric you choose. A moby wrap generally has a stretchy fabric that is good when baby it small, but makes it very difficult to use as the baby gains weight. A wrap is also something that you can easily make yourself. I would just look for a fabric that is breathable but doesn't have a lot of stretch to it, especially two-way stretch.
Best of luck.
If you want more product-specific info, I would check out www.thebabywearer.com