For me, there are a handful of games that I really enjoyed BITD (e.g. Arcade Space Duel, 2600 Asteroids) and that I still play regularly.
Otherwise, my favourite genre is RPGs. It is a chance to try the many, many titles that I read about in magazines, but that I lacked the hardware to play them. The quality varies wildly, of course, but I am not obliged to play the less fun titles; indeed they tend to get deleted quite quickly.
The vast majority of the ROMs in my "collection" are for games that I actually want to play. There are few platformers and no sports games because I just do not care to play those types of games. I am therefore not overwhelmed by endless choices.