well, I only tryed GD and ImageMagick.
and personaly I prefer IM, because it supports all formats.
GD1 doesnt support .png pictures and GD2 doesnt support .gif.
but, GD2 works way much faster then IM, with same quality.
GD1 create thumbnails terrible quality.
there is a thread with poll: "witch one is better: GD or ImageMagick" u might want try to find it