[Freeze]

I personally own:

Magpul PTS Gen 2 PMAGs (from the olden days)
-these were a bit loose in my VFC, I put a piece of velcro inside the magwell now they are snug.

Magpul PTS TMAG (transparent version)
-these are just slightly wider than the PMAGs

Dytac Hexmags
-with the 3d printed followers from Blackblitz these feed excellently

King Arms Metal STANAG
-works, fits snugly in my VFC

VFC Grey Metal STANAG
-literally the tightest thing I've ever tried to shove into anywhere. I had to smack the bottom a few times to get it to seat.

MAG Mag midcaps
-bought from Toronto Airsoft they're the $10 or $15 polymer ones, they work pretty nicely, fit, feed, but feels cheap (because they are)

If you're looking for lower capacity don't. As most people above have said, just get a midcap and underfill.