I was given a Moby and a classic Ergo at my baby shower for my first. He was a July baby and we regularly have 100F+ days here in summer, so, well, it was HOT. I mostly used the Moby in the early weeks when he was still curled up in the fetal position and happy being snuggled in. The Ergo w/ infant insert offers the same feeling for baby, but it was way, way too hot at that point. After the first little bit though, I stopped using the Moby. I found it to be a pain to take out and about because I hated the idea of the ends dragging on the ground of the parking lot when tying/untying. We then started using the Ergo without the infant insert but used the little pillow the insert comes with to lift him higher up. He was happy in that until about 4 months when he really wanted to face outwards. I didn't have a front-facing carrier, so we just carried him in our arms during that particular phase. Before long he was happy to face in again. I never really liked the back carry on the Ergo. He didn't seem to love it and so I never got enough practice being comfortable doing it solo. The last time I used the Ergo was when he was about 22 months and we were traveling. He's really tall but skinny for his age, so while I was OK with the weight he wasn't loving the fit much at that point. We also have an Osprey Poco Plus hiking carrier that he loves, and we use that now that he's bigger for hikes, traveling, etc. (We aren't big stroller people.)
I've just recently traded my classic Ergo for the Ergo 360. A few friends that are on their second babies had the classic with #1 and upgraded to the 360 with rave reviews. I was able to sell my old one for the same price I bought a used 360, so I went for it.
I'm also considering getting the K'tan this time and selling my Moby. They seem to have the same feel for baby, but with less fabric to deal with. I think the soft carriers are nice in the early days but a more structured carrier will last you longer. Similar to the Ergo but with similar reviews within my mommy-friend circle is the Tula.
A friend loved her ring sling, but I never tried one. My kid started walking early and wanted to be down on his own unless he was snoozing or nursing, so I found a more structured carrier suited us better.