Currently, despite my love of the PSP, the GBA SP reigns as the greatest handheld of all time. Why? Because it's SMALL, it has OMG BATTERY LIFE, it's like a SNES SQUARED, it has an INCREDIBLE LIBRARY INCLUDING THE BEST OF THE SNES PLUS ORIGINAL CONTENT and IT ISN'T GIMMICKY. It's pure 2D 16-bit era pleasure with a killer screen and a near-perfect form factor. It's just so cool to snap open and close, and it's completely unobtrusive!
Some NUMBERS to establish the FACT of its awesomeness versus the SNES which some of you creepy Ninthings inexplicably worship:
CPU: 16.8 MHz 32-bit ARM7TDMI with embedded memory
Memory: 32 kilobytes + 96 kilobytes VRAM (internal to the CPU), 256 kilobytes WRAM (external to the CPU)
Resolution: 240 x 160 pixels
Sound: None 
Color support: 512 simultaneous colors in character mode and 32,768 simultaneous colors in bitmap mode
Backward compatibility for Game Boy and Gameboy Color games is provided by a 8.4 MHz Z80 co-processor
sure beats the SNES, max resolution and lack of a sound processor aside (which is why GBA ports always sound a little bit worse than the SNES originals:
CPU: 3.58 MHz Motorola 65C816
Memory: 64 kilobytes VRAM, 128 kilobytes WRAM
Resolution: 256×224 pixels or 256×240 pixels (the most commonly used video modes)
Sound: 8-bit Sony SPC700
Color support: 256 simultaneous colors in character mode and 32,768 simultaneous colors in bitmap mode
Both consoles have hardware chipset support for various graphic functions like sprite and background alpha (transparency), color addition/muliplication. The GBA allows for sprite and background matrix operations such as rotation/scaling (and supports quite a few more sprites overall) and some simply geometry operations, whereas the vanilla SNES only supported background scaling/rotation (no sprites). The GBA also supports MUCH bigger ROM memory addresses (i.e. cart sizes), which means more animation, more color, and larger content.
But fuck the specs! The GBA SP rules because it has the best form factor of any handheld, as well as the best library! You can hide that motherfucker in a pocket, and unless you have fingers the size of bratwursts, it's comfortable as hell.
ALL PRAISE THE GBA SP. If you don't respect, GEDDA FUCK OUT.
