Buying comics can be a pretty tough job now. Not only are there hundreds of different series, but most of them consist of one long narrative from start to finish.
Gone are the days when you could just pick up a random issue without needing to know what had come before it, the general storytelling in comics has now evolved to the point that you'll be lost if you haven't been following the plot from the beginning (or even sometimes the plot of a completely different series).
However, while a lot of comics simply serve as a single piece of an ongoing narrative, there are some issues that stand on their own as an individual accomplishment. These comics could be ones that tell a whole story in a single issue or ones that do something interesting enough with the medium that you can enjoy them regardless of how many of the previous issues you've read.
This is far from an exhaustive list of all the best issues of comics (y'know, since there are literally thousands), but it is here to provide a great place to start for stories about iconic characters and to show off some of the amazingly creative ways a comic can be presented.