Very difficult question (I take it that you mean not song in itself but Adele's cover of it), but I think I would have to say Lovesong.
Second place would be Many Shades of Black (not fully cover considering that Adele was doing vocals with the Raconteurs) and Promise This. On the same second place ;) would be I Need You Now with Darius Rucker and Video with India Arie. I think the duet version of the later is more interesting from the original version, which itself it very good. Adele's and India's voices "melt togehter" wonderfully.
On third place another not fully cover, Water And Flame with Daniel Merriwether. I really like it. I think it's a song that really goes well with Adele in a similar way as Many Shades of Black, although both fit more the 19 mood than one of the 21.
As for I Can't Make You Love Me and Make You Feel My Love, they are not "my" type of songs, but like very much Adele's interpretation of both of them.
"20 bucks Adele takes home best movie tonight at the Oscars..."