From @Queuetue : The Derringer is my choice for testing, in a mix or with a single flavor. Please support creative people by buying authentic atomizers, but I also keep a number of these clones from Fasttech on hand so I always have a an atomizer ready to go while I'm mixing.
From @Queuetue : I find boston rounds to be the very best bottles for mixing and temporary storage. I use them for my VG and PG as well as on the scale while I'm making a mix. With a top that's easy to fill, easy to cap and with minimal access to air, I keep a bunch on hand for unflavored base, or whatever project I may be up to for handy, quick access.
The tip is small enough to use with most top-fill RTAs or RDAs as well.