26 gauge is ideal for most attys if you aren't looking to go very low or high ohms with your builds. It's probably a good idea to also get one size higher and lower just to have some options. 28 gauge is useful for dual coil RTAs if you want to stay around .5 ohm. 28 or 30 gauge 316L wire is good of you like making twisted wire (works very well in TC mode if you space the coil) but it's crazy springy. I also recommend trying out a spool of
26 gauge 304 SS. It's much less springy than 316L, which makes it easier to build with. 316L isn't actually difficult, you can use a slightly smaller rod than your target size to make coils (use a 2mm rod to get a 2.5mm coil for example). 26 gauge 304 SS it rock solid. No spring in that stuff.