I have two answers for this... My first cigar that gave me an epiphany was a Tatuaje Havana VI Verocu #5. Beautifully oily, it was the first "decent" (Non-cheap) cigar I smoked, and the first time I discovered (for myself) cigars could have flavours beyond tobacco.
The second was a SCDLH El Principe. That was the first time I discovered cigars could have desert style (or sweet) flavours.
There were cigars in between those, but those are two that stick out as being eye opening!