I think that the Arbiter is the best. Its Recall is awesome. Being able to cloak units around it. And also its ice attack(i can't remember what its called) is perfect. Stopping units from moving or attacking while your units are cloaked and takin them out. Although the arbiter's attack isn't good its a great spell caster. Carriers are alright. But they're very expensive. And take a long time to build. Although they're attack makes them partly worth it. Most people aren't going to waste time and money trying to get them when a lot of other units can take them out quickly.
dammit it i clicked wrong.....i was gonna click shuttles but hit observers....reason for shuttle.....fast, reliable....and holds reavers......reavers wipe out ground.....and you put 2 reavers in the back of a base guys minerals are shitted thatZ all
carriers are good but no one has said anything about observers! theyre spies and i use them to as many every vespene geysers and mineral patches i can! if u stop other people forn getting them by countering any expansions they send u have a good chance of winning. i use scouts more than carriers though arbiters are good when u team them w a few carriers.