I've got two favorites...
Metallica - 2004-10-19 - Wachovia Center - Philadelphia, PA, USA

I've always liked Metallica, but seeing them live just put them way higher on my list of bands. People might not like them very much, but they definitely put on a good show... especially considering the massive audiences they play to. Usually at shows this large, I leave feeling like I hadn't even been to a concert... not the case with Metallica. Awesome.
Jason Mraz - 2004-03-03 - World Cafe, Indre Studios - Philadelphia, PA, USA

Un. Freaking. Believable. Me and Melissa got there really early, so we sat (yes, sat... in a comfy couch I might add) front row. This was a syndicated radio show recording, so it was a different experience, to say the least. Jason played something like 6 or 7 songs, and did long interview segments in between. That boy can SING like you wouldn't believe... if you've heard his album, it doesn't even begin to do justice for his voice. And he's freaking funny on top of it. I went there barely knowing his songs (Melissa had won the tickets two days before so I scrambelled to download the album and try to get to know the songs) and came out a fan.
And these two (the only others I went to) are honorable mentions...
Maroon 5 - 2004-04-30 - College Union Ballroom, Gettysburg College - Gettsyburg, PA, USA
Great show, but the teenyboppers definitely subtract from the awesome factor. The annoying chatter in line then screaming in concert is enough to make a person want to blow up the entire planet. The band's ACDC cover was awesome to see in person, though. In conclusion, love love LOVE the band, HATE their fans. Oh yeah, also, this was the absolute loudest concert I've ever been to. Mostly because we were standing right in front of the speakers on the right side of the stage, but regardless I left with my ears ringing up a storm, and I couldn't hear anything.
Green Day - 2004-10-29 - The Liacouras Center, Temple University - Philadelphia, PA, USA
Green Day was beyond amazing as always. Still one of the best live bands I've seen. I felt really old at this concert, though... something that's never happened to me at a GD show before. Regardless, was a good time.
^ see what I did there with the colors? Eh? Eh???

God I'm a dork.