If you look at the number of clicks, the top 5 web feed readers are (it's interesting to note that Yahoo is the only one that displays only the titles):
1. My Yahoo 54%
2. Google 21% (Reader + Personalized Homepage)
3. Bloglines 11%
4. Netvibes 9%
5. Live.com 3%
In terms of post views, the situation changes a little bit (Yahoo couldn't be included here because you can't read posts inside My Yahoo):
1. Google 59%
2. Bloglines 33%
3. the others had 3% or less.
So the most important online feed readers are My Yahoo, Google Reader/IG and Bloglines, although it's hard to make a comparison between Google and Yahoo from this data. Google has more subscribed than any other feed reader, though: 76% from all FeedBurner feeds have at least one Google subscribers, unlike Bloglines (63%) and Yahoo (51%), which don't have a wide coverage.
Google and Bloglines seem to attract more tech-oriented users, but also more diverse users than Yahoo (hence the feed coverage). Google tries to satisfy both users who read a lot of feeds (and would choose between Google Reader and Bloglines), and users who subscribe to a manageable amount of feeds (Google Personalized Homepage or My Yahoo should be a perfect match), so it's in the perfect position to become the number one destination for reading feeds, if it's not already there.