During the short period that my parents actually paid to get HBO, I didn't really watch anything on the network, except for an occasional episode of Fraggle Rock. However, the awesome bumpers they used before the movies definitely piqued my interest, possibly even more than Fraggle Rock did.
This one from the 80s is probably among the most memorable. Yes, I'm talking about that awesome outer space one. They used this well into the 90s. To this day, I think that's the coolest thing ever. Plus, it's neat that HBO has kept the same logo after all these years. Most of the other cable networks have changed theirs at least once.