1. Alliances Unraveled- This deck brought a lot of staple attack cards. If I had to guess I would say a lot of top tier decks have almost 50% of their attack deck from Alliances Unraveled. Airsault and attacks like it are extremely good base attack cards. The set also brought us past creatures, and High Muges that would change the whole masters meta play.
2. Turn of the Tide- This set was a lot like AU in the fact that it brought a lot of base attack cards- but this time more focused on disciplines than elements. Poison Spheres and cards like it are a staple. The set also brought some other really good cards like Evergreen Tunic, and some good cards for Mril decks.
3. Forged Unity/Dawn of Perim probably tie this one up for me. As far as creatures go, Dawn of Perim was a great release; however, attacks, mugics, etc. were pretty weak as far as far as depth goes. Forged Unity was great because it at the time broke the boundaries of Loyal decks. Before Forged Unity, every deck was its own tribe. Forged Unity really changed that- and this change was really healthy for the game.
Marrillian Invasion would come down here because it released the Marrillians and some other really awesome cards like Tangath Toborn in training. It was a healthy increase of competitive card=play from the sets before it(in simple terms the cards were more useful by just the right amount).
Lastly we have Silent Sands and Zenith of the Hive. They make last on my list because essentially they were a dry addon to Dawn of Perim Secrets. As much as I love Blazvatan- overall the decks were lackluster in cards and in strategies compared to their counterparts.
When it comes to how I built my list- it came to depth of play from each set. I really took into consideration how much more depth each set added to the game. I consider this the most important feature for a set release.