My idiot brother scratched my Madden 08 game. It is unplayable, won't even go to title screen. I had it buffed at my local gamestop, which did remove all fine scratches, but failed to fix the one scratch that won't allow it to play.

Is there anything I can buy that will...fill in or in some other way fix a scratched disc?

I was thinking of buying SkipDr that I have heard some good things about, have you of you used this before?

buy the game again

A new disk.

The Game Doctor (possibly another name for the Skip Dr.) has a lot of positive feedback. If that doesn't work, buy another copy of the game, with the warranty. Then return your scratched copy and return the replacement- a lot of work, I know.
If all else fails, punch your brother. He knows that he deserves it, and you'll feel much better!

Don't use the store-bought Disc Doctor things, they don't work so well....my boss used to have this disc buffer, super high speed, he was able to fix every game I ever saw him buy from anyone except ones that had what appeared to be knife wounds in them. I had several unplayable games, and he was able to fix them, they look almost new.

EDIT---and sorry to contradict people saying to buy the disc doctor, but I own one, used it a few times, it seemed to work short term, but the games it "fixed" seemed to crap out totally soon after.

DOUBLE EDIT---On a side not, while we're talking about busted games, I tried to take my original copy of Tekken 3 out of it's case about 2 years after I got it (had been playing it all the time of course) and as I did it basically exploded, minimum about 8 pieces. All I could do was stand there and look at the pile of pieces, and about a minute later, commence with expletives. 

I can go to the store and they fix it for 5$. (Something that I just found out a month ago).

Strange, sorry to contadict you I guess. I've had to use a Game Doctor three times successfully. Twice with minor scratches, and one that was scartched over the whole surface (I never let my friend forget that he did it). All three were fixed, and ran just fine. The most damaged one, was on my PS2. The game came out a year after release, and was damaged a couple months after that. I'm still able to throw it into my PS2 today, and it works beautifully.

I think I will pick up the GameDR (aka SkipDR). The motorized version is $40 in retail and only $25 on Amazon. It seems to have a lot of good reviews. Plus, if this scratch is too deep to be fixed, my kids have a dozen or so DVD's that have minor scratches that I am sure can be fixed.

I will let you all know how it goes.