2015 Fnatic is the best western team of all time. The only other even remotely considerable candidates are Moscow 5 and S3 Fnatic; neither of which come even close to the combination of domestic domination and international success of the S5 Fnatic roster.
Moscow 5: Sure, they stomped the western scene but underperformed at Worlds and IPL5, losing to TPA.
S3 Fnatic: Just like S5 Fnatic, they won two EU LCS splits back-to-back and reached semis at Worlds. However, they didn't do well at IEM and their domestic record makes them look better thanthey actually were, with Fnatic looking quite shaky and inconsistent at times.
Now let's take a look at S5 Fnatic: New roster with Yellowstar the only OG Fnatic member remaining; the team is inexperienced and inconsistent at first; Huni, Reignover and Febiven are quickly turning into superstars; the team is improving fast and becoming the best team in EU; Fnatic wins the spring split; Deilor turns out to be an amazing coach; they go on to have an incredible performance at MSI, nearly taking out SKT; Rekkles is back; Fnatic goes 18 - 0 and takes the #1 seed at Worlds by beating Origen 3 - 2 (the other EU team to reach semis at S5 Worlds); at this point everyone on Fnatic is respected internationally and adored by fans; Worlds S5 - you know the rest.