Mad Dogg- Deco has been outstanding for Barcelona this season. I didn't see much of him in last season's champions league because I wasn't paying that much attention to Porto until the final stages. He was good in the European Championships as well, but wasn't Portugal's standout player.
Each of these players was their league's best in the 2003/2004 season. Arsenal and AC Milan both made the quarter finals of the Champions League and Henry scored 5 goals in the tournament to Shevchenko's 4. Barcelona weren't even there. Henry didn't do a whole lot at international level, Shevchenko and Ronaldinho were restricted to appearing in world cup qualifiers, where both performed pretty well. It's hard to split them really, so personal preference counts for a fair bit. Ronaldinho loses some ground considering Barca won no trophies and performed poorly in Europe, but I still think he played better football than anyone else this year.