Varies with how you interpret the term generation. Gen Y typically ranges from 1981 to 1996 or 1977 to 1994 for demographers. If we stick with the former, there's no room for Xavi, Ronaldinho, Henry and the likes. I'd probably rank the Top 10 as:
1. L Messi
2. C. Ronaldo
3. A. Iniesta
4. A. Robben
5. P. Lahm
6. L. Modrić
7. Neymar
8. Z. Ibrahimović
9. L. Suárez
10. D. Alves
Followed by the likes of Kaká, Ribéry, Neuer, Fußballgott. Neymar could very well end up as a Top 3/4 player for that generation over the coming years, especially considering his Brazil career in terms of being THE point of reference.