TLDR: an MFT (or a diy MFT-style table).
I have a decent sized shop with all the stationary tools: table saw, band saw, miter saw, router table, small drum sander, dust collector, jointer planer. I've also invested a fair amount in Festool the last few years. I started with a Domino 500, and that truly was an eye opener. Then I added a track saw, and found it much easier to break down sheet goods. And then I went a bit crazy and decided to go all out Festool: a bunch of sanders, a jig saw, an OF 1400 router, LR32, CT36 dust extractor, Domino XL 700, Seneca, and the list goes on.
But the one thing that really changed the way I work in the shop? A DIY MFT-style assembly table (using the Parf Guide System to drill the holes). Plus a bunch of accesories: TSO, Parf, Qwas, Bench Dogs, clamps from Festool and elsewhere. Using that table with dogs and fences and clamps and stops, along with the track saw and domino(s) and guide rails and parallel guides and dust extractor and sanders and... has truly changed the way I approach a project. I'd say that I cut my table saw usage by at least 50%. With no sacrifice in accuracy. Dominos have improved my joinery immensely. Sanding is still a chore, but so much easier with sanders specialized to particular tasks. And everything is aligned perfectly square and stable using the MFT-style assembly table.
So if you're dead set on buying Festool, start with a Domino joiner. But if you want to improve your productivity: start with an MFT (or MFT-style) bench.
Jeff