Well I love all moments and I haven't really given this a thought but here are the first 10 I really like and came to my mind.Ok they're not in order, I just wrote them as they came to me xD

1.the Post-it wedding.(so cute~~~)

2.the McProposal.(just perfect <3)

3.Inters hot-dog eating contest.

4.Season 4 finale.(extraordinary ^_^)

5.Season 1 episode 2 MerDer elevator scene.

6.Mer on morphine.

7.MerDer in the bathtub. (HAWT~~~)

8.MerDer at the prom.

9.Derek,Addison,Mark,Burke and the chief at the elevator when the Chief had died his hair.

10.Season 3,episode 13 the MerDer scene where Derek says to Meredith that he is always going to show up.

I have tons more that I love but you said only ten so here goes xD

Just of the top of my head
1. Izzie/Alex wedding
2. The last episode of season 4
3. Meredith on morphine
4. The scene in the elevator when the Chief died his hair
5. The dog eating contest/the conversation at lunch about the woman with spontaneous orgasms
6. The scene when Callie asks if Mer and Cristina were ever confused for lesbians
7. The MerDer tequilla, getting caught in the car in Season 1
8. Elevator proposal scene
9. The scene in the elevator in Season 2 when meredith says she misses Derek and he says he can't
10. The Lexie freak out after eating eggs and after going through the Chiefs files on everyone at the hospital

There are probably more and possibly better but i can't remember rigth now (Oh and honorable mentions to when Doc is the dog from hell)

I just have to add the scene with the guy who watchs porn so he won't be in pain. Just brilliant!

Oh wow I never thought picking 10 could be this tough...

10. Mark hitting on Cristina
9. The angry dart-throwing at Joe's
8. Meredith's panties on the bulletin board!
7. Meredith high on morphine
6. Hot dog eating contest
5. Derek and all his women in the elevator
4. Merder being McNasty in the Exam room xP
3. The Post-it McWedding
2. Season 4 finale- the 'extraordinary' scene
1. The McProposal *sigh* =)

oh and the Merder bathtub scene, the patient watching porn scene, the camping scene...and more

10.Mark hitting on Christina /of course lol xD/
9."I miss you" - "I can't" Meredith and Derek in the elevator.
8.When Finn and Meredith lunched /3.3 I think/
7.Burke and Christina's wedding xD
6.When Owen kissing Christina - season 5's first episode
5.When Mark said: "Somehow, somehow I lost..." You get it I think.
4.McProposal <3
3.Mark and Addison's sex /3.14/
2.When Derek go to Addison's hotel and see Mark half-naked.
1.Season 5's finale...I cryed but I think that's one of the best scenes. ....