Wow. This was and fun to read! GMD is one of my favorite Disney and I love MLP, so when I found this story, I just had to read it. And I'm impressed.
The casting was pretty spot on, with only a few OOC moments, but very few and in between. Pinkie was a lot of fun(who wouldn't expect that from Pinkie?), Twilight and Spike worked nicely off each other, and Applebloom was perfect for Olivia. I honestly had a hard time not hearing Vincent Price's voice during the Discord scenes, but I still think he was a good choice for Ratigan. Even some of minor roles were good. Such as the Flim Flam Brothers, Trixie, and the Diamond Dogs as the henchmen, Gilda as Fidget, and my favorite, Granny Smith as the old mouse.
If I one complaint, its that you didn't really the songs, specifically World's Greatest Criminal Mind. But, it didn't ruin the story in any way. Still love it.

Love this

I knew you stole Q's line, it is going to take more then a fall to kill Discord, only reason he didn't flash his way to safety or try to float is because he was surprised and too enraged to do it, though the fall probbably cleared his head.

20% bloodier works, it shows that under the mask of a gentledraconequus, Discord is a monster.

Bam Pinkie!

It's not hard to imagine Pinkie being amath genius, she's just usually easily distracted, but it's been shown time and time again, that she is very competant when she puts her mind to something.

Damn, Discord discorded Pinkie into depression, damn he's good.

It is fun to be make to sense to screw with others, but when things make no sense for you, less fun. Maybe you should have Discord say "I enjoy making no sense, but this is ridiculous."

They have the list... and as for not being able to put paraphrased songs in your stories, that's bull! Of course you can!

Damn Gummy must be big...

This will be fun...

This should prove to be fun.

Discord as Ratigan? This should prove fun...

:D love this! And, between you and me, the great mouse detective was Disney secretly ripping off Sherlock Holmes.
But whatever, this story is still awesome!

This was a great story! I loved "The Great Mouse Detective growing up and you did great using the Pony characters for this. :) Personally though, I would've used one of the diamond dogs (Mainly Rover) as Figet, but I still liked it. Good job! :)