There are many kinds of modems, because there are many different technologies used to provide an Internet connection. Which one are you using?
By "modem" you could mean dial-up telephone (POTS), or any of a variety of ADSL, cable TV, fiber-optic-to-the-premises like AT&T UVerse, T-1, or many other types of modems. These vary around the world depending on the technology that the various Internet service providers use.
So which "suggested one" model of modem for a Time Capsule would depend entirely on which Internet service provider company you are using, and what kind of method of connecting you to the Internet you are paying for (dial-up telephone, DSL, cable TV, fiber optic, satellite dish, etc.), and ultimately what country in the world you live in. This is not a question with a simple answer.
Apple doesn't make modems, because there's no way that Apple could make all the many different devices that would be required to work with all the different Internet service providers all over the world. There are some Internet service providers that won't even let a customer choose or purchase their own modem for certain kinds of services; they require that you rent the equipment directly from them.
All you need to do is to use whatever method of connection you have to your Internet service provider (the "modem"), and read its manual to find out how to put it in "bridge mode" or "DMZ mode" so that you can connect your Apple Time Capsule or Apple AirPort directly to the modem, and have all your computing devices connect to the Apple Time Capsule's router.