Favorite Cameo Battle

In many games nowadays their making strong boss battles that relate to different games. Such as with Star Ocean III, even though the two games are made by Tri-Ace they mixed the games together in a sort of way by bringing in 2 of the playable characters from Valkyrie Profile as chibi-formed bosses in the secret dungeons. How many of you enjoyed fighting cameo battles in most games and which ones were your favorites?

Hmm... I can't think of any cameos I've seen that were boss battles. I've seen cameos in cutscenes (mainly in Breath of Fire), but not as a boss. I played some of Star Ocean III, but never got as far as the secret dungeons. When I eventually pick it up again, I'll have to keep an eye out for that.