There are some things that could be measured objectively, for example, the percent of top 10 pages in each engine that use cloaking to boost their SERP while subjecting surfers to obnoxious ads. Or the number of times redundant scraper sites appear in the top 100 results. There is general agreement among users of search engines that such results are of low value.
But overall search engine quality will always be subjective, just like the list of the 100 hundred best films, or the books that every educated person should read. How could anyone objectively rank 10,000,000 pages on "yellow widgets" and say that this page is the best, this one is second best, and so on? It can't be done objectively by people, much less by a general algorithm.
"Best" and "better" imply a perspective or set of criteria for judging, which must be either subjective or arbitrary. How much should spelling errors ding an entry? When are more words better, and when is a brief, concise description preferable? Should a technical article written for college students be valued more than a general article on the same topic that is written on a 6th grade level?
Search engines will change, and people will use the one that meets their needs best (or is heavily advertised with the catchiest jingles).