English is rather complex phonologically. Lots of vowels for starters, and if we're talking about American English these include the rather rare R-colored vowels - but even without them things are pretty crowded, e.g. /æ/ vs /ɑ/ vs /ʌ/ ("cat" vs "cart" vs "cut") is just one big WTF to anyone whose language has a single "a-like" phoneme, which is most of them. Consonants have some weirdness as well - e.g. a retroflex approximant for a primary rhotic is fairly rare, and pervasive non-sibilant coronals ("th") are also somewhat unusual.

There are certainly languages with even more spoken complexity - e.g. 4+ consonant clusters like "vzdr" typical of Slavic - but even so spoken English is not that easy to learn to understand, and very hard to learn to speak without a noticeable accent.

You never realize how many weird rules, weird exceptions, ambiguities, and complete redundancies there are in this language until you try to teach English, which will also probably teach you a bunch of terms and concepts you've never heard of. Know what a gerund is? Then there's things we don't even think about that challenge even advanced foreign learners, like when you use which articles: the/a.

English popularity was solely and exclusively driven by its use as a lingua franca. As times change, so too will the language we speak.

English is far from the most complex or difficult.