when i started playing guitar i wasnt to into it i just thought it was fun, but i would have to say the song the inspired me to start playing a lot was the first time i heard "Pull Harder on the Strings of Your Martyr" by Trivium
haha this is where every 14 yearold says " NO BOUNDRIES BY BATIO" or "ARPEGGIOS FROM HELL FTW"

but seriously closed casket requiem by the black dahlia murder helped me the most asin it gave me the basic techniques to be able to play that genre.
It all depends on what catagory your referring to when you say, improving. In terms of speed, Constant Motion - It helped me understand alternate timings better, it helped me look at arppegios as something to experiment with, not just copy like so many guitarist do. It also opened my eyes to different positions of Harmonic Minor scales and most of all...just super fast shred!

But before that, when I first started playing, there was Yellow Ledbetter by Pearl Jam - That song encouraged me to use my little finger more and helped me to understand the theory and dilberate descision that was involved when it came to jamming by feel, instead of just learning endless techniques.

And of course, Master or Puppets - The ultimate technique improving song!
Yep, Master of Puppets. Learnt it about three months into playing guitar. Took me a while to get it all but it was worth it.
blindfolds aside by protest the hero
it was a great leap up from playing pop-punk stuff
I actually had to move around the neck
plus it has a pretty easy tapping part
I wish I could tap more, but the strings are so close together on my bass
Lord Gold feeds from your orifices and he wants to see you sweat.
Lord Gold probes you publicly and makes your pussy wet.
Now say his name.....
Well, in terms of soloing I'd say Fade To Black. The in and out solos in the song were the first couple that I really sat down and had hours of frustration and hardship with. lol
Serrena by Jason Becker. Just when I thought I had the sweeping down... **** was thrown in my face.

Ibanez SAS36FM
Ovation Acoustic
Classical Guitar
Amplitube 3 running into a PA