1. A yarn that knits at 3 1/2 sts per inch. Not 3, not 4. For instance, the wrapper on Cascade's Eco+ says the gauge is 3 1/2 sts per inch, but in reality, it looks better at 4. Similar problem with Quince's Osprey; the label says it works at 3 1/2 sts per inch on US 11's, which should be clue enough to tell you that it really knits better at 4 to the inch on a US 9.
2. A wool that isn't superwash. If you don't know this already, superwash is unpredictable after washing. It can grow ridiculously. Often manufacturers tell you to put it in the dryer (horrors!), for 15 minutes, then lay it flat to dry. The trouble with that is that you lose all control over shaping, and the fibre loses body and goes limp. Case in point--Ella Rae Chunky Superwash. I knitted a swatch in it a couple of years ago, and the swatch grew to almost twice the size of the original.
3. A wool or wool/natural fibre blend. I want warmth and elasticity, even lightness. I loved Elann's Peruvian Highland Chunky, now no longer available. It was lightly spun, airy, and perfect, apart from the fact that it came in 50g balls, thus requiring a lot of splicing. I don't want too much alpaca in a blend, because alpaca makes a heavy garment.
4. A soft yarn, so that collars won't itch against the neck.
5. Lucious colours, either solids, heathers, or tonals. It seems that all the otherwise suitable chunky yarns out there are space dyed.
So, why am I on a quest for the perfect chunky? I've just finished knitting this.
The yarn, Galway Chunky, is lovely, but it falls down a bit in the colour department. The button makes up for it, though.
Celtic horses, from The Ram's Horn Studio, purchased at Rhinebeck a few years ago. The knitting was fun and quick. This is really a new, unstructured version of Petrova, so the pattern will be quick to write up. If only I could find the perfect chunky to go with it...
P.S. I ought to have added price as a criterion. Few knitters can afford to make an entire sweater with yarn that costs $20 a skein!