After I learned paradise city by guns in roses, i felt an immediate increase in my guitaring ability. Its not hard, but not easy. The movements between power chords in one of the main riffs is tough to master and the solo is long and hard to learn. The solo is not that advanced, but it really taught me a lot more about how to put notes together.
Right now i am working on van halen's eruption (going pretty smoothly so far)

I want to know if there are any really good songs to learn to increase your ability.
Ability of what? Make sure when you're covering songs you listen to yourself back and make them perfect, not a half arsed copy. And write your own songs too . And to answer your question - play songs you like.
Just start out by playing some BLS if ur a Zakk fan...

OR since ur into older rock, play some Randy/Ozzy stuff

if ur a rock fan ur either a Randy or a Wylde fan..few exceptions
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