I think if you take the complete intersection, the set is empty. However, the five standard vowels /a/, /e/, /i/, /o/, /u/ are fairly common (although many Native American languages have four vowels). For consonants, /p/, /t/, /k/, /m/, /n/, /s/, and /l/ are probably the most common.
Esperanto's phonotactics is basically Zamenhof's Slavic dialect (Polish and Belorussian). However, Lojban tries for a more neutral disposition, even to the point of permitting epenthetic vowels in complex consonant clusters (stressing that they need to be distinguishable from the 6 regular vowels--the 5 vowels above plus /ə/).
It should be noted that when languages have restrictive phonemic inventories, the challenge is not so much in producing the other sounds as it is in recognizing them as distinct sounds.
Esperanto's phonotactics is basically Zamenhof's Slavic dialect (Polish and Belorussian). However, Lojban tries for a more neutral disposition, even to the point of permitting epenthetic vowels in complex consonant clusters (stressing that they need to be distinguishable from the 6 regular vowels--the 5 vowels above plus /ə/).
It should be noted that when languages have restrictive phonemic inventories, the challenge is not so much in producing the other sounds as it is in recognizing them as distinct sounds.