Nokia 6230.
I can't pinpoint one best feature, what's good about this phone is the whole package. It's almost as compact and lightweight as a basic phone, yet very advanced: Great screen quality, camera, video with audio, EDGE "2.9G", triband, XHTML browser, MMC slot, Bluetooth, MP3 player, FM radio in earphones. And a slick design.
I got mine with a 32MB card and a Bluetooth dongle for the PC, which allows me to have lots of fun.
For example, I downloaded a sound file from the PC to the phone which has Worf saying "Captain... incoming message!". Then I set it as my SMS sound!!
I also have some silly MP3 ringtones.
Most annoying thing(s): Hmm, well, the middle button in the "joystick" is tricky to press which is pretty annoying considering it's the primary choice button. And Nokia's software sucks, which makes me use the crappy buggy software that came with the Bluetooth dongle.
And it costs a fortune.