I think that the Vikings are one of my favorites in Terran. They can be BOTH ground and air units, which can be very useful in many times. For example, a swarm of Corrupters are chasing your Viking. You can just shift it to ground mode and chase them back.
This is the picture of the Terran Viking in ground mode:
And this is the air mode:
And here’s the Viking in a skirmish with the Protoss:
For some reason, I miss the old Goliath. But I think that the Viking is a lot better than the Goliath, so I have nothing to worry about.