Leaving aside the 750 mL standard, what's your favorite size wine bottle? The 3, 4.5, 6, 9, 12, and 16 liter bottles are always up there for me, not only because their giant proportions give an 'Alice in Wonderland' feel but for their crazy names as well: Jeroboam, Rehoboam, Methusalah, Salmanazar, Balthazar, and Nebuchadnezzar. I don't know who decided that the large-format sizes of wine bottle should bear the names of Biblical kings, but it strikes me as deliciously absurd.
Fancy nomenclature and daunting size of the giants aside, I think my favorite would have to be the 375 mL half bottle. It doesn't have a funny name and it's a little bit stubby, but at a table set for two, it lends itself perfectly to pairing with meal courses. Sometimes it can be a real pain to match different plates with one kind of wine, and two full-sized bottles can be a bit much to drink for two diners who plan on leaving the table under their own power. Enter the half bottle. Start with a 375 of a nice Sauvignon Blanc to enjoy with the oysters, then on to a half bottle of Cabernet or something from the Rhone to go with your lamb... you see the possibilities. Even for a single course meal a half bottle can at times make more sense than a full sized one, if you've got to drive or just don't feel like two or three glasses each. Of course, at most restaurants there are usually wines available by the glass, and while this is a perfectly reasonable solution to the question of divergent tastes or desires at the table that obviate the possibility of sharing even a half bottle, there exists the possibility that the bottle your glass is poured from was opened quite some time ago and the wine inside has suffered because of it. A half bottle is opened at your table, fresh for you to enjoy.
Unfortunately, not every restaurant that stocks wine carries small bottles, so when I find one that does I make sure to thank them for doing so and encourage them to continue. It is something I would love to see more of.