most important effects:
filters, delay and reverb
next most important:
phasers, bit reduction, distortion (all varieties from overdrive upwards)
to justify having effects in rack they need to have a decent amount of modulation inputs and a decent amount of modulation sources to feed them though
one of the most important things for a guitarist/bassist who's using modular imo is to be able to use pedals to control the modular - with foot switches and expression pedals - so you can play the guitar/bass and wiggle the modular at the same time
I'm a guitarist too and whilst I don't have long or slender fingers - multiple small modules are unusable when next to each other - to be able to use a knob of any kind you need around 1cm clearance on at least 2 sides
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities