Haha, I've actually thought about this as well (watching a drama with every letter of the alphabet). :D
I thought
49 Days was a good drama. The ending left some viewers unsatisfied though. If anything, it's worth watching for Jung Il Woo as the Scheduler.
Other dramas I thought were good...
1 Litre of Tears - It's a tearjerker (guess the title makes that obvious), but such a good drama and based on a real story too.
Empress Ki - I loved this drama. It's long, but the episodes just fly by. It's very pleasing to the eyes and has such interesting characters.
I haven't seen any dramas starting with a U, V, or X. But I've heard various people say that
Ugly Alert and
Vampire Prosecutor were good.