[Serra Britt]

Let's see if I can make some suggestions based on stuff I have.

Chrono Crusade: Seems rather fast paced to me since I felt like a lot of stuff happened over the eight volumes. Might have a bit too much humor to really be considered mature though.

Rurouni Kenshin: Had a few slow spots but for the most part stuff moved along nicely. Definitely mature-ish since it deals with "value of life" kind of issues.

Cardcaptor Sakura: Not really much filler so I'd say probably fast paced? I always think of it as mature because it had some heavy issues in it, even if most of them had to do with love.

Those are the top three I'd recommend out of my collection anyways. Nothing new though since I haven't bought manga in years >.>