Making my own Awesome Xander list. These may or may not overlap with sosa lola's.
1. Although he started out extremely jealous of Angel, he let go of his hatred for him partway through Season 2. He only starting hating him when Jenny died, which is perfectly understandable, but then he let go of it again.
2. He forgave Spike in Season 7.
3. He kept Dawn company when no one was paying attention to her.
4. He climbed the construction ranks very quickly.
5. He helped Anya find her inner humanity after a millenia of evil.
6. He remained a Scooby even when he could have left.
7. He saved the world by being a friend.
8. He influenced Cordelia into dropping the queen bee act.
9. He's there to save the day when the villain doesn't expect it (smashing the urn when the First is targeting Buffy in "Sleeper").
10. He doesn't let the loss of his left eye bother him very much.
11. He brought Buffy back to life in "Prophecy Girl" when Angel proved useless, becoming the secret hero of Season 1.
12. He wears Hawaiian shirts.
13. He apparently taught himself to use weapons (in "Dirty Girls", when he shoots a crossbow perfectly).
14. He thinks he's a coward but will get everyone out of danger before he himself tries to escape ("Becoming, Part One", "Dirty Girls").
15. He bonded with Spike despite pretending that he didn't like him.
16. He loves Willow even when she's scary-veiny Willow.
17. Season 3, Episode 13, "The Zeppo". Enough said.
18. He's straight on the surface but subconsciously has the hots for Spike ("Intervention") and possibly Angel ("Teacher's Pet"). Well, that may not be a reason for awesomeness, but I like that about him.
19. His love for Jello can save lives.
20. He will never join the dark side. Lame last reason, but true nonetheless.