Via F13. (read the rules if you are going to post your own top 10). Now onto my list, with some explanations following each choice.
1. World of Warcraft - Millions, yes millions, of subscribers. Penetration into non-gaming media on a large scale. To me, there is no argument against WoW being #1.
2. Ultima Online - Developed before there was a real market, didn't copy anyone, and remains a unique experience. Oh, and player housing!
3. Star Wars Galaxies - Included for POTENTIAL. This was billed as the first game with the POTENTIAL to attract a million players. Sadly, it proved there are no sure things in this market.
4. Guild Wars - First mainstream title to go completely against the grain of the subscription model. Proved that it can be done, but more importantly, it can be sustained in the long run.
5. Dark Age of Camelot - Showed that timing and smooth launches are equitable to success in the market. Plus, DAoC proved that the little guy can get it done with a smart plan.
6. WWII Online - MMOFPS? Yes.
7. EVE Online - Another POTENTIAL inclusion. The game itself isn't spectacular, but the design behind it is begging to be turned into something great.
8. Lord of the Rings Online - I compare LotRO directly to Star Wars Galaxies. There are no sure things, even when the developers play it extremely safe. Lower than expected, only because THERE IS NO FUCKING MAGIC IN LORD OF THE RINGS TURBINE!
9. Everquest - Only mentioned for being brave enough to bring 3D graphics into the genre.
10. MUD 1 - The literal "birth of online gaming" can not go unmentioned.