WP RSS Aggregator is clean not only on the website but also in the WordPress back end. Adding feeds is easy and straightforward. Add the title and feed url and you're of and running. After trying six or so other RSS widgets this is the one that does it all for me.
When I did run into a snag the developer was very responsive and helped me get everything working the way I needed it to straight away. This is my RSS aggregator of choice. Try it!