Hmm...I think this is a rather interesting topic, because the definition of a niche title isn't really that simple. What's standard in video games has really changed as the market has grown and new hardware has become introduced and players become used to different things.
For instance, take a game like Actraiser on Super Nintendo. It was a interesting blend of action and strategy that felt new and different when it first came out, but if the game was released today, I can't help be feel it would be really mainstream, at least in a sense that it wouldn't turn too many heads, except for the fact that it plays well.
Looking at another Super Nintendo game, E.V.O. The Search for Eden. This game is still probably more in line with being a niche title, even in today's market. The ability to evolve your creature over different generations to survive, is still a fairly unusual idea in video games.
Then there are games that are considered niche when they are translated over to America because they are not frequently translated to English. For instance, the Playstation game Incredible Crisis is certainly considered a niche title here in America, but is it necessarily a niche title in Japan? Maybe so, but the hordes of dating sims that come out over there are not niche titles in Japan, but if any ever got translated to America the would certainly be a niche title here.
Then there are games that start out a niche titles, but gain enough sucess that they spawn franchises and become popular enough that it's kind of hard to call them niche titles anymore. Games like Dance Dance Revolution, Monster Rancher, and Harvest Moon come to mind here.
I guess what I'm trying to say is that niche titles are often hard to pinpoint because what is niche varies over time and from location to location. Yes, there are some games that I think will never become mainstream enough to not be niche, like Artdink gems like Carnage Heart and Tale of the Sun, but some of the games I have fondest memories of and thought were niche games at the time I played them, are hardly niche games anymore. I guess that's why it's hard to come up with a favorite here.
Feel free to chime in with your own two cents regarding this if you wish.
