I have a project where I want to attach 28 electromagnets to an arduino and control them separately.

What kind of electromagnet would best be used with an ardiuno in this case?

I want it to be able to be strong enough to attract ferrofluid on a vertical surface



You're going to need a driver board, electromagnetics will likely need more than a few mA to generate much electromagnetic force.
I offer a 32-channel N-MOSFET board that can sink 1A/channel, that should create some pretty strong attraction.
Shown here with a Duemilanove attached, it can be controlled from any microcontroller that can shift 4 bytes into shift registers.