Acropora - hardest, especially go grow colonies. Only a few get here, but many do OK for a while with frags. There are LOTS of different kinds.
Montipora - like Mushrooms... some are really cool ones like Bounces and Jawbreakers, but the rest grow like crazy and can get on your nerves after a while. These are really easy to grow. There are branching and encrusting varieties.
Birdsnest - grow like crazy and are easy.
Pocillipora - these can spawn and take over everything if you are good at reefing. Some people have a no Poci policy. They are also easy to grow.
Stylophora - grow like crazy and and are also easy.
Some people only keep Acropora and have a no MBP&S policy - I am one of these... just want to save the room for the acropora. I hate tearing out colonies of MBP&S and then have to give it away. The stuff is like weeds to me.
Of the acropora, there are all different kinds, but stags and slimers and usually the easiest. Tables, tenius (a species name) are in the middle. Millepora, Prostata (also species names) and the smooth-skinned deepwater species are typically the hardest.
When looking for starter stuff, look to the MBP&S variety and just about anything from ORA. Even the ORA acropora usually is some of the easier stuff.
Perhaps the most important thing to remember is two-fold... so two important things. The care requirements of Acropora are vastly different than those of MBP&S. Also, it is far easier to care for frags and mini colonies than it is to keep real colonies - the flow and lighting requirements change.
