Science Vessel is in my option the best since you can use Emp to shut down any protoss shield (How other people had stated) Defensive matrix to defend some important tanks (Yamato Cannon cant even damage enough a single tank to make it weak)

Gas ( other people stated making 200 lings would win but Gas can damage everyling around them like they were nothing that and we can have air to take down any ling running away  ;D