For me? Tileset probably, but personally I'd like to see an expansion of TF before getting a new one. (snow tiles, outdoor city tiles, better overworld etc.) If that's not an option, I'd rather pay more and wait a bit longer to get a solid set that can actually do something. (TF is quite good in this regard) We currently have one zillion packs in the store with one zillion different styles and themes but not a single one to make an actual game with more than one town and dungeon without having clashing art. Second I'd say battlers. It would allow us to keep a consistent art style for the game where it matters and give a lot more depth and versatility to TF. (On that note, gib monster pack nao!).
Lastly, facesets. Not to bash facesets, but I think those really aren't viable to focus on because they are incredibly inflexible. Some people want busts, some people want icons, some want everything and most of the time the pack doesn't have it and the facesets are commissioned elsewhere.
I have never used a faceset provided with a pack because they either have 5 characters with eight emotions or 20 characters with one emotion, all in a size I can't use. These options aren't viable for a developer, and I'm assuming you making 50 versions of 50 faces is not commercially viable for you as an artist either. (Especially since your art style is distinctively unique and does not go well with other resources provided by RM/Degica. But perhaps I'm wrong here and other marketplaces view it as hot pancakes.)
But that's just my not so humble opinion. In the end, go with what you think works best for you. And remind me to commission a certain corner tile sometime.