a matrix mixer - really handy for: getting more complex modulation from what you alreaddy have (mult 4 modulation sources, get 4 more complex ones out), feedback patching (mix output of xyz module back into input), send/returns (send outputs of modules into inputs of effects and then mix outputs of effects back into original signal - useful for effects modules that don't have dry/wet controls)
joranalogue compare 2 - great logic module...
I know what you mean about modules that already have xyz in them - but usually it's part of something else - taking useful functions out so they can be used on their own is a good practice - so you always have that functionality available, even when using the module for something else - Maths is a great example of this - it's really powerful in itself and especially when patch=programmed to do something more interesting - much more so than using it as the individual functions it is made up of - which can be had individually for a relatively small amount of cash!
Disting and O&C are similar - you can only do 1 thing (or 2 with the ex/hemispheres) at a time...
"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