If I could only have one…
I would go with a 24″-ish square clam shell, at least 9″ high.
24″ is big enough to accomplish most large single pieces or a good number of small pieces. While a “tub” is nice, I have a Skutt GM1414 and it takes a LOT of glass to fill it. It seems like a big waste of space and energy if you need to slump a larger piece that has a sufficiently different firing schedule that you cannot fire it with other pieces.
The clamshell is a little easier to load and it lets you easily get at the glass should you want to do combing or other manipulation while the glass is hot.
The greater height (9″ or 13.5″) will let you do taller drops and drapes for vases, or more height on a pot or mesh melt.
Get a square kiln, not octagon or round (more usable space).
The SKUTT GM22CS fits this criteria, though it is pricey. The evenheat studio pro 24 also fits my criteria. You might also consider a 16″ or 19″ Paragon clamshell, though they are only 6.5″ tall (but less expensive).
Just my 2 cents,
Jester’s Baubles Fused Glass Designs