I think every yarn has its place be it scratchy wool, super soft alpaca or stiff unforgiving cotton. Even eyelash yarn has its place. Can you tell I’m not a fan. But I have used it to knit hats for charity.
If I had to pick one yarn and no other to knit forever it would be Cascade Heritage. It’s a fingering weight superwash sock yarn that comes in lots of colors. I’d be able to knit just about anything I want.
But my most favorite yarn to try is Indy sock yarn. I let myself buy single skeins of sock yarn that I’ve never tried before most every chance I get. I love seeing the creative color combinations and then knitting them to see how they work up in different patterns.