What material should be used for large-scale electron emission?
Photo cathodes

Each material has a work function (actually a constant), which is the energy needed to punch out an electron so that it leaved the surface of the material. Here is a short table:

Palladium                 5.2 eV
Nickel                    5.0 eV
Iron                      4.6 eV
Copper                    4.5 eV
Tungsten                  4.3 eV
Cesium                    2.1 eV
Barium-/Strontiumoxide    1.0 eV


Now, the photo effect is a wonderful proof that light sometimes behaves like a ray of particles (photons) than like a wave:
The energy of a photon must at least be equal to the work function to release the electron from the surface. If it has more, the excess will be converted into kinetic energy of the electron.
If light were a wave, you just had to wait long enough until an electron has accumulated enough energy to leave the surface. 

The energy of photons of visible light range from 1.6eV (red) to 3eV (violet), so it can not release electrons from the first materials in my table. 

This is the reason why fancy materials are used in photo cathodes. It's also clear that something like Cesium must be placed in a vacuum tube, because it is very reactive.

"Electron gun"

If the goal is to generate free electrons, other techniques are used. The simplest is heating a piece of metal until it glows, as it it used in vacuum tubes or CRT screens.
The heat also gives electrons enough energy to escape the surface, and the hotter, the more electrons you get. Tungsten is a great material as it melts at about 3000°C, while something like Cesium is difficult to handle, melts at 28°C and is a rare (expensive) element on earth.
Though the work function plays a role here, too, it can easily be compensated by just more temperature. 

Another method to generate free electrons is field emission. Here, strong electric fields are used to literally pull out electrons of a surface. Extreme fields can be obtained with very sharp tips, and today, we use arrays of tips with the width of just a few atoms. Though again the work function plays a role, it is more important to have materials which allow to build this tips. The production is related to the production of semiconductors, and similar sets of materials are used. 

Ionizing of air

Air is ionized  by applying a high field again, until a spark or a corona is formed. Heat can play a role, but more as a by-product. In addition, ionized air can be a quite aggressive chemical, and the material has to withstand it.
One very common use case for ionized air (or more: sparks) are spark plugs. The tip is usually made of nickel or nickel-alloy, as this is not only heat resistant, but also quite resistant to the aggressive ionized air / fuel. More expensive plugs have a more needle-shaped tip, which is covered with palladium or iridium due to the higher temperature at the tip.