Omega: Sometime before the Citadel Coup attempt by Cerberus, either before Priority: Sur'Kesh or right after Priority: Citadel II (Meet with Councilor Valern/Esheel). Leviathan: After Rannoch, before meeting with Councilor Tevos/Irissa I guess it depends on variables like LI and characters you want involved for the Party but prior to the Citadel DLC the order was best as: