Tough to narrow this down to 20, I'd like to see pretty much all of these get made. But, if I had to make a list - not taking into account rumors/thoughts of what may possibly get made in the future - I guess this would be my 20:
1. Han Solo (Stormtrooper) - ANH
2. Luke Skywalker (Stormtrooper) - ANH
3. Grand Moff Tarkin (resculpt) - ANH
4. CZ-3 - ANH
5. Bom Vimdin - ANH
6. Luke Skywalker (Bespin) - ESB
7. Han Solo (Bespin) - ESB
8. Snowtrooper - ESB
9. Willrow Hood/ICMG - ESB
10. Hoth Rebel Soldier (SA) - ESB
11. Luke Skywalker (Jedi/Endor) - ROTJ
12. Hermi Odle - ROTJ
13. Warok/Lumat 2 Pack - ROTJ
14. Leia (Endor) - ROTJ
15. Galactic Marine - ROTS
16. BARC Trooper - ROTS
17. Mace Windu (Ultimate) - ROTS
18. Obi-Wan Kenobi (Ultimate) - TPM
19. Qui-Gon Jinn (Ultimate) - TPM
20. Jango Fett (Ultimate) - AOTC
That's a tough list - had to fudge a little by putting the woks together. There are others I'd really like to see though (Padmes, Lars, Lando Smuggler, Luke - Medical Frigate, Luke Snowspeeder resculpt, DS Trooper, etc.) - but limiting it to 20 and figuring we won't get them any other way - I guess this would be my list. I get more excited for OT figures overall, and from the prequels my priorities would be SA, "ultimate" versions of the main Jedi who really need them - Obi-Wan, Qui-Gon, and Mace. After that, the clones from ROTS, and then Padmes.