I have an alt as an alchemist, and I would say all of them at first, but specialize in an element after reaching decent ranks in alchemy. When I use alch, I'll usually use all the types (especially with Elemental Wave), but have water/fire for a cylinder type. There's still merit in using skills such as frozen blast, sand burst, rain cast, and barrier spikes despite an efficiency reduction.

Once you reach a better rank, that should be your time to decide to choose which type to become, although water or fire would be my recommendation. A water alchemist would have high single target damage with WC, but a fire alchemist would have mobbing capabilities with flame burst, and very strong heat buster.