Some of my favorites are the first VOG runs and the loot cave, or that very first weekly, where I was underleveled and my friends were trying and I were trying to do it without really knowing what arc burn meant. Those three coins were quite satisfying. A more recent favorite is when I went to get the Abyss chest (thinking "eh, may as well get some shard/energy before the weekly reset) on an alt for the first time and ended up getting Gjallarhorn. I was pretty happy about that.