He was in this form when he first met Joe in the digiworld.
Bukamon digivolves into his rookie form, Gomamon. He's easy and outgoing, unlike Joe. His marching fishes is his attack, funny and very effective.
Gomamon digivolves into this stage, his champion form. His attack is the Harpoon Torpedos and is usually used as a boat. Poor guy.
His ultimate form, when Joe's crest of reliability shines. His attack is Thor's Hammer, which is made out of a special digi-metal.
His mega form, which never really appeared in Digimon, but is on the battle cards. Really cute form. :-)