For me it was my hunter. I loved it since day 1. I've completed all three subclass quests, all hunter milestones in the AoT book, and got most of my exotics on my hunter, and transferred some to other characters. It might not seem like a big deal, I know. What character have you spent the most time on?
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Warlock from the very beginning. I hate the floof, but love everything else about it. I always leaned more towards the more scholarly classes in mmos. I don't like the mechanics of the hunter, and the Titan seemed too bulky, and linear in personality for me.