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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My Titan, though close on hours for every character, Titan looks the best and I feel plays the best, I have about 5 days on each, 4 on hunter, I've restarted some time on each.