Peaceful corals are quite passive in their existence and vulnerable to being killed by more aggressive corals. The corals below are all peaceful.
Blue Corals
Gorgonians
Soft Corals
Semi-Aggressive corals can (and will) fight for superiority with other corals in your aquarium. Be sure to give them the space they need so they don't feel threatened. The corals below are all semi-aggressive.
Gorgonians
Mat Polyps
Aggressive corals can dominate an area of the aquarium (or the whole aquarium) and kill off other adjacent corals as they grow. They are adept at defending their territory and will do so, so be sure to give them plenty of space to grow. The corals below are aggressive.

Heliopora spp
Diodogorgia spp
Pachyclavularia spp
Tubipora musica
Anthelia spp
Dendronephthya spp
Lobophytum spp
Sarco phyton spp
Sarcophyton elegans
Xenia spp
Acanthastrea lordhowensis
Acanthastrea spp
Blastomussa merleti
Blastomussa wellsi
Caulastrea curvata
Caulastrea furcata
Cynarina deshayesiana
Cynarina lacrymalis
Duncanopsammia axifuga
Echinophyllia spp
Merulina amplicata
Montipora capricornis
Montipora nodosus
Montipora spp
Tubastrea spp
Turbinaria peltata
Turbinaria reniformis
Turbinaria spp
Muriceopsis spp
Clavularia spp
Actinodiscus spp
Rhodactis spp
Ricordea florida
Ricordea yuma
Entacmaea quadricolor
Heteractis spp
Macrodactyla doreensis
Cladiella quadricolor
Sinularia dura
Sinularia flexibilis
Sinularia spp
Acropora efflorescens
Acropora millepora
Acropora spp
Alveopora spp
Catalaphyllia jardinei
Diploastrea heliopora
Favia spp
Favites spp
Fungia spp
Goniopora spp
Lobophyllia hemprichii
Lobophyllia spp
Pavona decussata
Platygyra spp
Polyphyllia spp
Scolymia vitiensis
Symphyllia spp
Trachyphyllia geoffroyi
Trachyphyllia radiata
Parazoanthus spp
Protopalythoa spp
Parazoanthus spp
Condylactis spp
Heteractis crispa
Heteractis magnifica
Stichodactyla haddoni
Stichodactyla gigantea
Euphyllia ancora
Euphyllia cristata
Euphyllia divisa
Euphyllia glabresens
Euphyllia paradivisa
Euphyllia parancora
Galaxea fascicularis
Heliofungia actiniformis
Physogyra lichtensteini