The problem with what most folks consider 'starter corals' is that while they're hardy and good growers, switching from these starter corals to more advanced ones can be difficult. Many of these produce noxious substances to inhibit other corals, or they're very invasive.
That being said, here's what I consider the 'starter' corals:
- zoanthids / palythoa
- green star polyps (GSP)
- mushrooms
- xenia
- anthelia
Then a little more advanced:
- leathers (toadstool, cabbage, finger, devil's hand)
Then the easiest LPS:
- frogspawn
- hammer
- torch
And then the easiest stony corals:
- montipora cap
- montipora digitata
- birdsnest