Whew, that took some counting!
Of my novels, I have:
14 group stories
10 Brian stories
6 Nick stories
2 AJ stories
2 Brian/Nick stories
1 Brian/AJ story
1 Nick/Howie story
0 Kevin or Howie-only stories
Before Sick as My Secrets, I had really only written a lot from Howie's perspective in 00Carter, which is a lot different from a normal fanfic because he's the supervillain LOL. So this is my first time really writing in Howie's point of view. I didn't start that story out of a desire to write about Howie; he just fit the role best, which is how I always decide which Boys star in my stories. That said, I am actually really enjoying it! I have probably enjoyed writing his parts more than Nick's, which has been surprising to me. Maybe there will be more Howie stories in my future!
I have never written a true Kevin story outside of one short story a long time ago, and I don't have any desire to write a story just about Kevin, but like I said in an earlier post, I do have an idea for a Kevin/Nick one. Again, just another case of me getting an idea that worked best with those two - yay Kevin!
As far as your last question about popularity, I would definitely say my Nick stories and group stories have been the most popular, but my results are also somewhat skewed because of how my writing habits have changed over the years. When I first started writing, I wrote almost exclusively Brian stories, and I averaged about a "novel" a month, so they racked up quickly. They aren't popular because they're super old and not well-known and also not very good LOL. Most of my Nick novels were written more recently and are a lot better in terms of writing, so it makes sense that they'd be more popular. Really, I've only written novel in the last ten years that was not about Nick or the whole group, and that was my Brian/AJ one, which was also an AU - I think that affects popularity too. That said, it wouldn't surprise me one bit if other people found that their Nick stories were more popular no matter what. Nick is The Chosen One LOL.