Not sure what you mean by modular cabinets. As far as I know, all cabinets come one at a time and have to be assembled to make them modular. Unless you are referring to IKEA cabinets, which I don't care for. Custom or semi-custom usually gives you better construction and materials, and more options in hardware and finish. Finish is a personal decision.
We are planning to renovate our kitchen. Is it better to buy modular kitchen cabinets or have them custom-made? What's an ideal finish for kitchen cabinets?
Most kitchens can be planned out with what you're calling modular cabinets with fillers where needed.
An ideal finish is one you like. Thermo foil may hold up better to cleaning, but if you want wood cabinets, it's not what you're going to get.
It might help if you defined what you're calling, "ideal".