Great question Steve! I just made up a little modular grid rack of what 6 modules I would pick:

https://modulargrid.net/e/racks/view/2611573

I decided to only include modules I actually own and use (otherwise the question is super open ended!).

For only 6 modules, I make the assumption that sequencing and effects would be done with non eurorack gear (e.g I would use my Keystep Pro to sequence and built in effects in my external mixer).

And I tried to focus on things that would really augment a non modular setup - i.e. things that wold be harder to do or you would less likely experiment with a non modular setup.

So I picked:

1. Error Instrument Noise Drum - a source of loads of wacky and weird percussion sounds.
2. Mutable Instruments Plaits - a digital oscillator with 16 different synthesis models. They are all really deep and I have still not explored everything this can do after owning it for 4 years.
3. Mutable Instruments Beads - a granular sampler - such a creative tool that can be used for weird and crazy delays as well as creating effects and massive soundscapes.
4. System 80 860 VCF - this is a Roland style filter - ok so this one you could get in a lot of places outside Eurorack and if I was allowing picks outside modules I own I'd probably pick something wilder/more experimental. However in actual use this my absolutely favourite and most useful filter that I use all the time and I thin I'd want it in my 6 modules.
5. Make Noise Maths - this is one of the most popular modules in Eurorack and is basically a super flexible device capable of so many things. Most normally I use it for envelopes and LFO style modulation but it can do a lot more.
6. Mutable Instruments Veils - quad VCA and mixer. Couldn't be without this to use both in modulation and potentially audio mixing and control.

NB: If you were actually going to get this rack - the Mutable Instruments modules are shown as no longer available because the company ceased to make them a while back. However because all the designs were open sourced there is a wealth of clones available and (for Eurorack) pretty affordably. The exception is Beads- which I don't think was open sourced and is now pricy second hand. If starting from scratch then I would swap this for another granular sampler from a different manufacturer. (maybe look at something by Qu-bit which have a couple of options in that area)

wow I wrote an essay. Thanks loads for the question and hope you're doing great!