There are a number of flags with cute indications of their country indicated on the design - Cyprus has the map, Kiribati has that nice sunrise and birds flying effect, Barbados has the trident. I think this is the best representative of that type of thing - cedar trees are indisputably Lebanese, but it's stylised rather than realistic and makes a nice colour scheme.
Of the pure rectangular flags (most of which are tricolours arranged vertically) I like this one most - somehow putting black at the top is cool, businesslike, subtle yet smart. The all-green Libyan flag was also a contender in this category, and I think the Bulgarian tricolour is also rather tasteful. Another near-miss was Greenland, with an interesting use of circle-and-line.
In a world where everyone else is rectangular or even square, the Nepalese have made the flagmakers' lives difficult by sticking to the traditional pennon shape - indeed, by making theirs a double. The contents are rich in symbolism as well - the triangles are the Himalayas and the motifs represent Buddhism and Hinduism. I hope the Maoists don't change it now that they are in power.
I occasionally drive past the Venezuelan embassy in Brussels and find the spray of stars in the middle stripe very cheering and uplifting. (See Colombia and Bolivia for much less successful uses of the same basic colour scheme). I must confess that I had not realised that an eighth star was added in 2006 until researching this post.
It's no secret that Macedonia is my favourite Balkan country; their flag is my favourite Balkan flag as well. Changed to meet the demands of the neighbours in 1995, the Macedonians somehow moved from a rather dull display of a disputed archaeological artefact to this rather brilliant sunburst, which should occasionally lift their instinctive pessimism.
Interesting that two people separately asked me for this list! More later.