Best way to style first post differently?

I think you can add an class to the [post_class()][1] function via Filter; the name, the string can you use form different values: tags or post title?

The follow example use the title of post and add this to the classes of post; if you use the post_class() function on post tag in markup. Also you can use other values for design this.

function fb_title_post_class( $classes ) {
    global $post;
    $classes[] = sanitize_title_with_dashes( get_the_title( $post->ID ) );
    return $classes;
}
add_filter( 'post_class', 'fb_title_post_class' );