SC2 was the best in my books. It came right at the end of the arcade era in my area. I found it to be, by far, the easiest one to come to grips with as well. Ivy, for example, is an absolute cinch to learn to use in SC2 but has gotten progressively harder as the series has gone on, imo.
SC3 and 4 were good. The customisation options were fun, but they were console only. The whole reason I even had SC2 on PS2 was to practice for the arcades. I still bombed out of the little tournies that the local arcades set up - my guard impacting has always been far too reactionary (i.e. GI-ing on misguided instinct forgetting that opponents will often mix-up or use holding variations of certain moves to throw the timing), but it was fun.
As for SC5 - I perservered with Pyrrha and Patroklos for a while, and I felt proficient enough with them, but they weren't Cassandra.

I remember when I first got into the game - I went with Nightmare (which played more like Siegfreid in SC2 days), moved onto Kilik before finding a permanent main in Cassandra. All three characters were great for ring-outs which was my favourite way to win. As I got better, I found out that Talim played in an annoying poking, grab-happy style could beat most people in my area and that successfully pulling off Ivy's Summon Suffering was the equivalent of the FPS teabag in a fighting game.

Yes, lots of fun indeed.