I prefer apps that have consistent GUI with apps shipped with maemo 5 (except for Ovi Maps which look quite different and are rather diffucult to use). That holds especially for widgets, they all should really have the dark grey background -- I like the look of:
Desktop command execution widget
Personal dataplan monitor
Foreca widget

OMweather - little different, but okay

I dislike the look of:
nametag widget - the widget works well, but I took it off my desktop because it simply does not fit in..