It's 2003, so 13 shows is a pretty respectable showing. What comes before the beginning of anime? Apparently quite a lot. Even though Naruto technically started in 2002, it really gathered momentum here in 2003. This was the best year in terms of quality that Naruto ever had. It's far and away better than any other portion of the series. Meanwhile, this is also the best season Inuyasha has, so we're looking at a pretty strong 1-2 punch here:
1. Naruto - Zabuza arc ending + Chuunin Exam arc beginning
2. Inuyasha S4 + S5
3. Full Metal Panic? Fumoffu
4. Scrapped Princess
5. Uuchuu no Stellvia
6. Prince of Tennis - Hyoutei arc
7. One Piece - Jaya arc + Sky Island arc beginning
8. Da Capo
9. 12 Kingdoms 2nd half
10. Galaxy Angel S3
11. Hikaru no Go tv ending
12. GITS S1 2nd half
13. Macross Zero ovas
Scrapped Princess and Uuchuu no Stellvia are both sci-fi space stories. They're also both unforgettable tear jerkers. No list of great anime can ignore 2003 so long as these two shows are laying around. Prince of Tennis's Hyoutei arc is also the best portion of this franchise. Full Metal Panic? Fumoffu isn't the best season of FMP, but it sure is the funniest.
Da Capo begins this year to rave reviews and sales. However, its true glory won't be until Da Capo II.
This may look like a weak list with just 13 entries, but with so many outstanding series at their apexes, I believe this year is actually better than 2004 or 2005. If you haven't seen this section of Naruto, you just have no idea how good anime can be.