What's your favorite load-out for Stormfiends?

I've faced Stormfiends a few times, and my opponent has 6 of them, one with each weapon option. He built them that way simply because that looked coolest, but I don't think it's actually that bad an approach. I know the usual advice in WFB is to go for specialism - you don't want an expensive shooty unit wasted in combat, or combat monsters having to hang round while their mates shoot. However, a big part of the threat of Stormfiends is their versatility. When they turn up (usually in the most annoying place possible - and you really have to take one with Grinderfists - it's easily their strongest feature) you don't know if they're going to charge you straight off, or obliterate you from distance. No matter what you try to desperately manoeuvre to engage them, they have a tool there to deal with it.

So I'd say their loadout doesn't matter too much, and going for the "one of each" option is actually pretty good. However, if you want to focus them, pick either shooting or combat, and select accordingly. I think you should always have one Grinderfist SF in there for the Tunneller rule regardless, and the combat unit is probably nastier as your Doomflayer Gauntlets and Shock Gauntlets get the 4+ save to keep you mostly safe from small arms fire. Being able to charge after appearing is so nasty too. There's not much between Doomflayer Gauntlets and Shock Gauntlets.

Or magnetise, as Vulcan said. Rob Hawkins did that with his, if you're confident with your modelling skills: robhawkinshobby.blogspot.com/search/label/Stormfiends

They are an end of times unit and are in the 4th book released, the Skaven one (and Sod’s law, I can remember the names of all of them except that one).
Edit - Thanquol

Each one has to take the same weapon option on both arms and the chest, but you can have different models in the same unit with different weapon options, so you could have one each with Grinderfists, Ratling Cannons and Doom Flayer Gauntlets for example.