The bonus chapter where GLaDOS gets into an arguement with Trollestia. This troper barely ever laughs at humor in books or fics, but this got me to lmao for a good hour.
The best part? The Mythology Gag. One of GLaDOS' most common insults against Chell in Portal 2 is fired back at her in a most brilliant instance of comic irony. "...Are you calling me fat?" Yes, she did, and GlaDOS' reaction makes it even better.
Zig-Zagged somewhat. Sure, the way she tricks Fluttershy into killing the cube is funny (and would probably be outright hilarious if done in the games). Switches to Dude, Not Funny! when you suddenly realize that was Fluttershy she did it to.
Pinkie Pie putting herself in portal-induced free-fall. For at least ten minutes.
Rainbow Dash's introductionto Chell. Fluttershy bought Chell a piece of cake, and while walking down the street with it, Dash crashes into Chell and causes the cake to fall onto Chell's head. After being told about the Enrichment Center's appearance in Equestria, Dash flies up to check it out, accidentally punching through some rain clouds and causing it to rain on Chell. Then, when Fluttershy offers Chell some of her cinnamon roll to make up for the lost cake, Dash returns and causes the cinnamon roll to land on Chell's head as well, followed by the realization that she punched through some more clouds on the way down. The Death Glare Chell gives Rainbow Dash is earned many times over.