FR Deoxys is pure offence. Insane Attack, Special Attack and Speed stats, but absolutely laughable HP, Defence and Special Defence. Pretty much any attack will take him down providing you get the chance.
LG Deoxys is the defensive version, but not quite as dedicated. It has brilliant Defence and Special Defence, and only average attacking abilities. I like this version best for its general all-roundness compared to the others.
RS Deoxys is a toned-down FR version. Slightly lower Attack, Special Attack and Speed. Slightly higher Defence and Special Defence. Unless I'm mistaken their HP is the same.