I'm like Brenda, I use them all.
But if I had to run back into my burning house for only ONE, I guess I'd take a Maya wrap LP ringsling. Newborn through toddler, it would always work, albeit not always 'best.'
It sounds like you just need a different type of MT- maybe one with wider straps. My first MT was actually a MT hybrid- a Babies Beyond Borders. It was perfect for my growing baby. I was attracted to the price point and the wide shoulders, with a structured waist. Mind you, I ended up liking them so much that I now do customer service for BBB, just so you know.
There are so many great MT's out there- www.attachedtobaby.com
is a great place to shop, or just c'mon over to www.thebabywearer.com