8
Best to worst (Purely subjective)
Imagemagick - goes with a standard install of Linux, so it should be supported on most reliable systems. For some reason it doesn't always work though.
GD 2.0 - Not as many options like image edit, rotate etc. Most hosts have it.
GD 1.0 - Just an older version, works, mature, virtually everywhere.
netpbm - Experimental, most hosts do not have this, have to install yourself (It is the best, but not fullt supported and still cumbersome)