Why thank you.
Modular world is not unlike guitar pedal world. Although I can recommend my favorites (marshal guvnor! EQD Hood!) ... I can make a much more informed recommendation if you give me some guidelines as to what you want to use it for. Monosynth with filter? Drum machine on steroids? Stereo sound processing world? Point me in a direction.
Monosynth with filter, maybe something with more than one VCO but a tasty filter is a must.
Ok. That does however put you down the wormhole for all the components of a single voice (IE an oscillator, envelope generator, vca, filter and input/output module. Do you have a ballpark budget? There are all in one solutions like the makenoise 0-coast and moog mother32 that are great starting points and come with an all in one version of these core modules and is still patchable to additional eurorack stuff you add later.
Tasty filter comes in so very many forms and prices. For my money the synthesis technologies e440 is the goddamn greatest version of the filter in the prophet 5, including the original, and gets aggressive enough to make it MVP most versitile player in my rack. For what it's worth. I also LOVE the SEM filter Dopher makes (the original is only in the SEM itself).