This is an easy question for me for the #1, but for other positions, it's harder.
I first heard
Just Dance and liked it. Put it on my iPod later, still love it 3 years later.
My #4
I didn't really heard about
Poker Face when it came out

but liked it later. A song that I heard SO much in clubs and partys, so
I love it.
#2
Love Game was one of my favorite song when it was released. If you have a chance to listen to the instrumental, it's fabulous. I started to like Gaga more and more at this time. [B]#6/B]
Paparazzi : I didn't really like the song as much as the other ones. But! The video just killed me. When I saw it, I was so wow-ed. And I got really interested in her. I started watching all her other videos.
#8
Bad Romance : I liked the demo at first, more than the original. When the video came out, I couldn't believe it...I played the video and listened to this song a LOT for something like a year, requested it when I went out in clubs and everything. When I shuffle on my iPod and those "oh oh oh oh" starts, I just can't skip...
Definitely my #1 Gaga song and video, by far.
Telephone : I really loved the song, it's my type of song. Loved the video, was very humoristic. But, one month after the video was released, I came back to Bad Romance.
#5
Alejandro : The song is good, different from the other ones. The video killed the song though. Again, one month after the video was released, I came back to Bad Romance.
#7
Born This Way : I loved the song at the first listen, which is really rare for me. I kept loving it more and more at each listen. The video wasn't near Bad Romance and didn't make me love the song more, but it wasn't bad. Great song -
#3 song.