2. What is your most expensive manga? Assuming that we're talking about individual books and not boxsets, then probably the Azumanga Daioh omnibus (from Yen Press). The retail price was $24.99 according to the cover, although I know I didn't pay that much. Still, I imagine I still paid around the $15-20 range for it.
3. What was your least expensive manga? The manga I got for free as gifts, obviously! But sticking to manga I paid for myself... Well, I actually bought the two volumes of The Flower of the Deep Sleep off the Amazon Marketplace for only a cent each, but I still had to pay shipping on them, so they ended up being $4 each. Still a pretty good deal, though not amazing. Hmm, I vaguely recall paying a dollar (or maybe $2?) for a manga on clearance at Half-Price Books, but I'm not sure which one it was... Call Me Princess, maybe? Yeah, I think that was the one. It's something I probably would have never picked up unless it was that cheap. (Just checked the price tag that's still on it -- $2.49, but I still don't think I paid that much for it.)
4. What is the most boring manga you own? Miyuki-chan in Wonderland -- pure fanservice, and not even that sexy. Still, it was CLAMP, so...
5. What is your favorite manga series? Fruits Basket! Although if NANA was ever completed, it might knock FB from the top spot.
6. What is the most relatable manga series you own? Kimi ni Todoke, definitely. I identify with Sawako like "woah"!
7. What is one manga you own that is based off an anime? As in the anime came first? Umm... I don't think I have any. I guess the closest would be Story of Saiunkoku, since the anime and manga were both based off the original light novels. (And I'm pretty sure the anime came before the manga.)
8. What is your rarest manga? Well, I don't know about rare in the general sense, but one of the most difficult volumes for me to find was Name of the Flower, Vol. 1, since I had the misfortune to order it right before CMX went out of business. I searched for around eight months for a decently priced used copy. And I still don't own print copies of Red River Vols. 15-17, but I have digital copies, which is good enough. (If I found those print copies for really cheap, I'd probably pick them up, but I'm not going out of my way to search for them.)
9. What is the most reprinted manga you own? I have no idea, really. I guess if you interpret the question to mean which had the most editions, then Sailor Moon might count: chapters in Mixxzine/Smile, Chix Comics editions (the single-chapter floppies), the Tokyopop graphic novels, and the new Kodansha graphic novels. (And I actually own most of those.)
10. What is the most popular manga you own? I imagine it's either Fruits Basket or Death Note, depending on which demographic you're talking about.
11. What is the most damaged manga you own? No question, the Tokyopop GNs of Sailor Moon. I've got pages falling out of a few of those.
12. Which manga has the most amazing art? I'd say A Bride's Story has the most amazing art in a technical sense -- Emma from the same mangaka is really beautiful as well -- but my heart really lies with Yuu Watase's style. CLAMP is no slouch themselves, especially when Mokona's the lead artist, like in Card Captor Sakura and Magic Knight Rayearth.
13. What is the oldest published manga that you own? Well, I know that Sailor Moon was the first manga I bought, but I don't think it's the oldest published, especially if going by when it first came out in Japan. Looking at the dates at MyAnimeList and Anime News Network, it looks like the oldest would be Let's Stay Together Forever, which came out in 1989. (Have no clue about American publishing dates.)
14. What is the newest published manga you own? Looks like Vol. 3 of Yukarism. (This time, talking about what was released in the U.S.)
15. What are some of the most recent manga you have purchased? The aforementioned Yukarism Vol. 3, Meteor Prince Vols. 1 & 2, Utsabora omnibus, Skip-Beat! omnibus (with Vols. 31-33), Arata: The Legend Vols. 21 & 22... Those are the ones that I remember off the top of my head.
I tag anybody who wants to do this!