It depends on what class of service you are considering and what amenities and services are important to you. For coach, if you want a more seat pitch and AVOD, go with NW, then KL, then, to a lesser extent, AF. Avoid DL and CO. If you want free booze, go with AF. If you want consistently good service (for the most part), do not care about AVOD, can endure poor seat pitch, and want to pay for booze, go with CO.

For business class, there are many threads and discussions in this forum that compare the Skyteam J products.