what are the best ferry lines for island hopping
The best ones are those that get you where you want to go from where you are, and in a timely manner on the day you want to travel, but since you haven't offered up any information there's no way to really advise you. I suggest you start by researching your route and use this ferry schedule website to help you determine your best plan of action. Keep in mind that every time you move to another island it takes time to accomplish, starting when you check out of your hotel, make your way to the ferry port, travel to the next island, find your new hotel, check in, unpack and get familiar with your surroundings. Don't try to see too many islands in the time you have. Relax and enjoy. Greece is a wonderful country. Have a great time!
We had a good experience using the large Blue Star ferry line. They are a bit slower, but have a smoother ride than the tiny fast ferries and also have less chance of being cancelled due to weather.
I just realized I forgot to include "this ferry schedule website"!!
Sounds like you've been through this process many times. We're planning to fly to Santorini from Athens and take a ferry back. I tried asking a travel agent in Athens to help us make sure we get ferry tickets, and they wouldn't help me. I didn't book a hotel or travel package through them. How hard is it to get ferry tickets? Well be traveling back to Athens on Sunday, July 15th. Should I order them before we get there?
July might be a bit busier, but generally booking tickets to leave an island when you arrive works well. As Lee and another poster mentioned the main difference is large/slow vs small/fast ferry, if weather is nice and no wind, the small ferries are fine, otherwise schedule and route determines what is best.
As for "hopping" plan to stay on any island at least 2 nights, 3 is better. Also, the ferries from Santorini (or any island) travel a set route, so pick islands on that route. Just because islands look close, that does not mean they have ferries between the islands, sometimes getting to a relatively close island would mean returning to Athens (Pireaus) and back out. From Santorini, I think there are two common routes that stop at islands along the way (you can get to other islands but heading away from Athens) Play around on the ferry websites and see what sounds good. Common recommendations would be Naxos, Paros, and Mykonos or islands on the Western edge of the Cyclades (Fologandros, Milos, Serifos)
Because it's so busy in Greece in July I do recommend that you buy your tickets at least a week in advance. That's usually enough. There's a travel office in the Arrivals Hall at the Athens Airport where you can buy them or you can order them through the ferry company website to be picked up at the departure port.