Actually, strictly based on the Loyalty Missions and not the characters:
1. Jacob
2. Kasumi
3. Mordin
4. Tali
5. Samara
6. Garrus
7. Thane
8. Zaeed
9. Miranda
10. Jack
11. Legion
12. Grunt
Honestly, ranking 3-8 was really, really tough. You could change the order real easy.
--
TheRock ~ Slow dramatic zoom-pan. Doesn't phase the hooded man.