Peak f(x) was 2011 2012 2013. Let's look at what they did in those three years.
2011:
PINOCCHIO becomes a smash hit and wins f(x) their first of many music show wins. HOT SUMMER is even bigger hit selling over 3 million copies. f(x) also is voted the Military favorite girl group (beating out SNSD!)
2012:
ELECTRIC SHOCK takes Korea and YouTube by storm and eventually becomes only the 3rd girl group music video to reach 100 million views (after SNSD and 2NE1). f(x) is so popular they are invited to Hollywood to film a special collab video with superstar Anna Kendrick. Countless kpop fans say this collab was their first introduction to kpop.
2013:
RUM PUM PUM PUM and Pink Tape is released to worldwide acclaim and becomes one of only a handful of 2nd gen girl group albums to sell over 100k copies. Was the only kpop album to make it on Fuses Top 40 albums of 2013. And was later named the Best kpop album of the decade by Billboard.
When you take into the inflation of kpop since 2nd gen, these feats surpass anything Red Velvet has done.
This isn't a diss to RV. Just need to remind people occasionally how big f(x) were.
come on man, SNSD in their peak was 
