I'd say the most needed classes are Templar, Cleric, Chanter, and Sorc for a group. Templar is the standard tank class, but a good Gladiator can tank most stuff, just not quite as well. Cleric is the standard healer, and Chanter is a good off healer and potential main healer if needed, but their mantras are quite useful for every group. Sorc has probably the best sustained DPS, but more importantly they have the best CC. Any other classes work for DPS, but some kind of combo of those 4 will generally always make a good group.
If you go to aiononline.com, you can find server statistics and see a breakdown of % population per class. Gladiator is the most played on every server, but all the RMT bots using Glads affects that probably. Sorc is generally #2, with Assassins being 3 and the others fairly even.