The discrepancies among various global university rankings derive us to compare and correlate their results. Thus, the 2015 results of six major global rankings are collected, compared and analyzed qualitatively and quantitatively using both ranking orders and scores of the top 100 universities. The selected six global rankings include: Academic Ranking of World Universities (ARWU), Quacquarelli Symonds World University Ranking (QS), Times Higher Education World University Ranking (THE), US News & World Report Best Global University Rankings (USNWR), National Taiwan University Ranking (NTU), and University Ranking by Academic Performance (URAP). Two indexes are used for comparison namely, the number of overlapping universities and Pearson’s/Spearman’s correlation coefficients between each pair of the studied six global rankings. The study is extended to investigate the intra-correlation of ARWU results of the top 100 universities over a 5-year period (2011–2015) as well as investigation of the correlation of ARWU overall score with its single indicators. The ranking results limited to 49 universities appeared in the top 100 in all six rankings are compared and discussed. With a careful analysis of the key performance indicators of these 49 universities one can easily define the common features for a world-class university. The findings indicate that although each ranking system applies a different methodology, there are from a moderate to high correlations among the studied six rankings. To see how the correlation behaves at different levels, the correlations are also conducted for the top 50 and the top 200 universities. The comparison indicates that the degree of correlation and the overlapping universities increase with an increase in the list length. The results of URAP and NTU show the strongest correlation among the studied rankings. Shortly, careful understanding of various ranking methodologies are of utmost importance before analysis, interpretation and usage of ranking results. The findings of the present study could inform policy makers at various levels to develop policies aiming to improve performance and thereby enhance the ranking position.