I started buying the volumes in 2000, when they started being published here, because i heard about half a year prior of this series coming out.
The first volume looked not that promising, but i was done with DB and was anyways searching for some other series, so i still remember first time seeing that cover and being sceptic, haha.
Going through the volumes, i had a long time volume 11 as a poster hanging in my room as a child, but i adored how Luffy looked on volume 10, so that i even drew him back then.
Also volume 10 and 12-15 i couldn´t find anywhere, so that i had to order them directly from our manga publisher.
Before that i had for about half a year no new OP content which made me almost lose sight of it, but then i discovered volume 16 in a small store and my father ordered with me the missing volumes. Don´t know how many times i read the first few volumes during that time, hehe.
I also remember that i saw Volume 13 in a small picture preview (while i studied the ordering list) for the first time and it was so small that i always thought it would feature someone with a huge brain^^ (which turned out to be Luffy´s round body)
Volume 18 with Ace stunned me the first time seeing it, couldn´t believe how cool he looked^^
Volume 21-22 i also had a poster of, which combined those two, just awesome.
Reading it back then i couldn´t see how Luffy would be able to defeat Crocodile and waiting 1 to 3 months for each new volume killed me :)
Volume 26 with Nami was also special for me for obvious reasons.
Volume 30 i hoped so long for, finally seeing Enel on the cover, but i couldn´t believe he wasn´t on the spine.
After that i more and more discovered reading the chapters online, which took away a bit of the magic of seeing for the first time a new cover and pondering about their content.
Covers i really liked were 39,40,51,57,58 and recently 77 and this one^^