I set my supplies up so they work with all types of resin without having to change things when I change resin types. With that said….
I almost always use #5, only because that what the 19oz cups I usually use are made with… but #1 and #2 work fine also.
ALWAYS AVOID #6!!! Unless you're going for that melted cup with resin look.
The dollar store sells plastic reusable cups that work great for dividing and mixing individual colors. When the resin cures, it peels out of the cup. They just aren't large enough for mixing all the resin without it splashing out of the cup…. Unless you're only pouring one or two blanks.