Vowels are much easier to me. I have problem pronouncing consonants that are back/glottal, like h in Slovak and many in Arabic. Besides, the r in Spanish and Russian is also hard. By the way, personally, I found tonal languages much easier. If I pronounce the tones right, it sounds good enough even if I ignore the intonation. However, for non-tonal languages, it’s hard for me to get the intonation. Intonation in French is still a mystery to me.