Which RSS and ATOM aggregator to use?

Depending on your needs, you could use something like Pluck or one of
the other "programs" to consolidate your RSS feeds. I prefer to
consolidate my personal feeds in "myYahoo" homepage. I do, however use
www.bloglines.com as a news copnsolidator for finding specific feeds
for www.dailyconspiracy.com as I feel bloglines is the best all around
web-based feed reader. I would highly recommend them.

I'm partial to FeedDemon http://www.feeddemon.com the first poster is
correct it is up to you. There are really 3 different ways you can
view or aggregate feeds.
1. In a desktop application, IMO FeedDemon leads the pack in this
arena and supports feeds with enclosures (like podcasts).
2. In a web browser, newsgator, yahoo and MSN all have web "start
pages" that can be customized with feeds in a browser.
3. As a plug-in this is where a small application would piggyback an
application like an email client and add the option of viewing feeds.

There is a complete list of RSS Readers (aggregators) segregated by category at:
http://www.rss-specifications.com/rss-software.htm

Best of luck!