Never really understood what classifies a set as an "Ultimate Collectors Series". Obviously there are those that have "Ultimate Collectors Series" written on the box, but there are others that do not, but still are classed by Lego as UCS sets (eg. 10188 Death Star, 10195 drop ship). There are also others that do not make the UCS classification by Lego but are considered UCS sets - or least they should be! (eg. Sandcrawler)
Others I think worthy of UCS status are (6210 - sail barge, for the cleverness of the design; 7665 - repulic cruiser, for being one the best looking models there is!)
Obviously I am looking at UCS sets from a Star Wars theme, since that might pre-dominant collection.