In World of Warcraft what is a good level 19 twink

Depends on your play style. Paladins can be overpowered sometimes, but at low levels like 19, they don't have a lot of attacks. Hunters can be good if done right along with rogues and mages. Priests are better for healing like druids and to a lesser extent shaman. shaman can be a twinked, but since their totems are not mobile, they might server better as base/flag defense than just smashing other people out in the open.

Just remember, if you twink, you will want to turn off XP. The drawback is that you will only fight other twinks who have also selected to turn off XP. This can make the que times extremely long.