While a traditional acoustic piano is the best instrument on which to learn, that's not always practical or feasible. Listed below are my top keyboard suggestions, in order of preference.
It is always ideal to start from day one with a full-sized (88-key) keyboard with weighted keys. Students who start off on smaller keyboards often get confused when confronted with the real deal. Additionally, students will need a full-sized keyboard and access to a pedal after one year of diligent practice. Making the investment of a bit more money on the front end will save you money down the road.
One final suggestion: good posture is vital in order to create good musical habits. Unless you have a table/stand and a stool/chair that puts your musician at exactly the right height, consider investing in a keyboard stand. Some of the packages listed below include a stand, and those that don't link to recommended stands.
Full Sized Keyboards:
Yamaha is a dependable brand, and I've never met a Yamaha I didn't like. ☺
Williams Legato III 88-Key Digital Piano Black 88 Key
If you go with the Williams Legato, be sure to buy the package that includes the foot pedal. Otherwise, you'll have to obtain that separately, which could be a hassle.
As with any purchase, be sure to read reviews, order where you can return if dissatisfied, and purchase the necessary protection packages.