I love the GAMEBOY pocket. It is one of my best electronic devices. It has such a big screen for the time, enjoy my review.
http://youtu.be/Gy27MKNgbdE

How do you post the video on the thread?

Game Boy Color, Game Boy Advance, Nintendo DS Lite. Those were the days.

Like this:

Based on just the form of the device, the Gameboy Advance SP. I loved the way games played on that thing. Still do.
However, I'd say my favorite is the DS lite because it was the first system I really started to get into from a hardcore perspective. I was getting games all the time, and there were so many great games for it. Plus, it was backwards compatible with GBA games, so I had a lot of fun with that too.

GBA Advance SP. "Wait, I don't need a flashlight? Wait, Circle of the Moon is playable now?"

gameboy micro, because when class gets boring i can usualy bust it out without getting caught cus its size makes it easy to hide

..After that I'd say GBA SP.

either the gameboy or GBA.

Gameboy colour for me 3 solid Legend of Zelda series entries are reason enough to make it number 1.

Does PSP count? It's about as old as the original DS. I'd have to go with that. Looking back, I have no idea why I got my PSP so late in its life. Its power is amazing for its age compared to the DS. Trails in the Sky, ZHP, Patapon, Monster Hunter Freedom/Freedom Unite/3rd... so many amazing games, so many hours of productivity sucked away.

Game Boy Advance SP

GBC. I loved that thing, even though the only games I liked for it that I had back then were SMB deluxe and the pokemon games. (I also had conker's pocket tales but I couldn't do much with it)

I wouldn't count it until it stops making Sony more money than the Vita. But if I did it would definitely be at the top of my list.

But yeah, Gameboy Advance. I still use that thing for all my non-PSP handheld needs.

Even though at the moment it seems that big is the new small when it comes to portable devices, I am of the opinion that the GameBoy Micro is the best handheld ever made. It's so small and neat and beautiful with a nice bright screen and obviously... it's ultra-portable! It's a shame it was so under-appreciated, but with the DS taking off around the same time of its release, I guess it was pretty unnecessary. Though whenever non/casual gamers see my Nintendo stuff, the GameBoy Micro inevitably gets a big reaction. Most people have never seen it before.

BTW do non-Nintendo handhelds count? I have a lot of retro handhelds that I have purchased with in the last few years, anyway here is my top 5 Retro handhelds.
1. Gameboy Color
2. Neo Geo Pocket Color
3. Gameboy Advance SP
4. Atari Lynx
5. Gameboy Micro

gameboy advance is my number 2!

GBA SP. My first handheld, and the first video game system I ever saw. I got a GameCube first though because of my brother.

Was never huge on the hand held scene, But if i had to pick i'd say the original GameBoy and the DS Lite. Sure, the original GameBoy is pretty akward to play in this day and age because the screen is so tough to see unless you have a lamp gizmo hovering over your head Inspecter Gadget style. But even then the motion smearing and monochreme pea soup effect weren't exactly stellar on the eyes. But the games that came out for the GB were phenomenal!

System wise, the DS Lite (Polar White) is still my favorite if you want to even consider that Retro. plus it's probably the most
whimsical and beautiful looking hand held ever created outside the Aqua Blue 3DS.

Original Gameboy...FTW