I guess I am an old-timer when it comes to manga, but I am a newbie regarding on-line forums and sharing my experience, so here I hope I am putting out the right foot forward.
I guess I have been reading manga for over 30 years, so I have started from a time when reading manga was taboo, and now reading manga is a hip thing to do. I started to collect manga when I became adult and had the mean to do so, and today I probably have about 2000 books in my collection (I never really count them, but Golgo 13 alone is over 150 volumes now). I bought them much faster than I read them, so I have only read maybe half or less of my collection so far. I have been working on reversing that trend in recent years.
I would say I stick to the main stream male titles in the adult and youth levels. There are probably more sci-fi themed than other categories. I haven't understood or appreciated yaoi manga, but I have enjoyed some of Clamp's work touching in this area. I generally would select a title more based on the art/drawing than on the stories (it is manga after all, not novels), and there are a few artists I followed religiously and I assume I have purchased every title they published.
Because I have a huge backlog, my expertise tend to be in the older titles instead of the up-and-coming ones. As a reference, the titles I am familiar with probably were published between the time of City Hunter and the beginning of Gunnm Last Order. Anyway, I look forward to exchange ideas with others who are interested in this type of manga as I do, and hopefully I can offer some information or assistance if applicable