Certainly Mongolian is the ugliest. I believe many people do not realize how ugly it sounds because linguists want to express it as a special language with special phonology and special grammar, in reality it is not so strict. The less formal it is spoken, the uglier it sounds. Casually spoken in the western Mongolia, it can be very ugly even if formal structure of speech is kept.
надад үзэг өгөгтүн [nɑdɑd uzeɢ ɵɢɵgtʰuŋ]
- give me a pen (most formal)
надад үзэг өгөөч [nɐdɐt ʉzɛɢ ɵgɵːt͡ʃʰ]
- give me a pen (more formal)
надад үзэг өгөөч [nɐdɐ̆t ʉzɘg ɵɣɵt͡ʃ]
- give me a pen (more casual)
надад үзэг өгөх [næð̠æ̆t yzɘx øʁø̆χ]
- give me a pen (most casual)
Necessarily ugly is not bad because then no one would want to speak at all less casually.