Off the top of my head.
One Piece Opening 15(?) - Fight Together
This one really keeps an optimistic mood throughout the opening, showing trials and tribulations of the crew, as well as demonstrating the good times. Moreover a first-hand look at what the meaning of "taking bad times to better appreciate the good times" is.
Baccano - Gun's & Roses
This theme gives me a sense of confident relaxation throughout the series, while display scenes that are less than admirable at times. The contrast with the jaunty, light-hearted music with malicious intent essentially creates intrigue within the series, and as such is appropriate to achieve the goal of what an opening is.
Sengoku Basara: The Last Party - Flags
A bit more simple than the other songs, but I think it's simplicity doesn't detract from enjoyment. It shows(rather than tells) the determination within each major character behind their goals, in a very stylized manner.
