Resi 2 on GBA !( But not released.)

Doom on my 2001 Nokia phone. It did brick the phone though :/

Quake on Sega Saturn was quite an achievement; Digital Foundry even did a video on it:

Agreed; MW3 especially was a stunning feat, I frequently found myself wondering "how the heck is this running on Wii?" during its more demanding setpieces.

Borderlands 2 on vita while not a great port by any means it was the perfect little pocket shooter. I had alot of fun with it anyway, shame they couldnt get that framerate locked

Always loved Super Mario 64 DS, I personally rank it amongst the best portable experiences of its era.

My buddy actually has a couple different video series on exactly this! He's done some awesome stuff regarding handheld games that pushed the system visually, and has a video on Driver 3's GBA version which looks amazing for the system. He also has a Max Payne one that I think did the rounds on YouTube for a bit. Whenever I see him talking about weird ports and such, it blows me away.

https://youtu.be/xXq3jH8TQNo for reference - the Driver 3 video!

Edit: To contribute more directly, the two Johns behind Doom porting Mario 3 to DOS, which was thought to be impossible, is always a personal favorite. But that didn't release so I don't know if that's fair to count. 

Here's some of the one's that blew me away.

Max Payne on the GBA

Doom on GBA

Modern Warfare 3 on the Wii

Metal Gear Solid 5 360 and PS3

Half Life 2 original Xbox

Meanwhile, on PC:
(Well, no. The game was updated on PC)

When I learned that Deus Ex was ported to PS2, I thought it was a joke. While the framerate is choppy, there is only one particular moment where it slows down to 1 frame per 1 second lol

I'm still floored by Tony Hawk Pro Skater 2 on GBA.

https://youtu.be/YyP5WocYg24