The current single coil go-to tank is still pretty much the tried and true Wotofo Serpent Mini IMHO. It's performance is hard to beat for an everyday single-coiler that can do both MTL or DTL, and it's pretty foolproof to build on relatively.

That said, being a mini tank, the capacity (2-3ml) might not be enough unfortunately. There's the Serpent Mini 25, but even that one isn't all that much bigger juice cap wise.

Just FYI, on the bleeding edge end of the spectrum, we have the Vandy Vape Govad AND the GeekVape Ammit 25... both single-coil poster childs of the "current" postless deck hype. Both are packed to the brim with features and perform awesomely, but might be a tad overwhelming to build on coming from a KF/KFL.

So yeah. If you won't mind the small capacity, the Serpent Mini is still the normally recommended single-coil RTA to upgrade to...

... unless you're feeling adventurous and wanna straight into the deep end that is.