We could say that the “proper” IPA values of letters from Latin script should be close the original Latin pronunciations of those letters, although those pronunciations aleady evolved even during the existence of the (Western) Roman Empire, and some symbols fell in disuse while other symbols were added through history.
It seems that Italian is commonly considered the modern language closest to Latin, in particular the (Logodorese) Sardinian dialect.
When I check the Wikipedia page of Italian orthography indeed it shows that the Italian pronunciation of written Latin letters is in great agreement with basic Latin letters used as IPA symbols.
One exception being “z”, which is pronounced as /ts/ or /dz/ in Italian, quite similar to the German /ts/ pronunciation of “z” and perhaps originating from the Ancient Greek pronunciation of “ζ” (zeta), although that may also have been /zd/ instead of /dz/.
Only “j”, “k”, “w”, “x” and “y” are not used in Italian spelling except in loanwords, although /k/ does exist in Italian spelled as “c”, /j/ as “i” and /w/ as “u”.
Many languages use the Latin letter “j” in spelling and pronounce it as /j/, such as Slavic languages, Northern Germanic languages, German, Dutch, Lithuanian, Hungarian and Finnish.
English, Romance languages and Turkish are notable exceptions to this.
Also, many languages use the Latin letter “k” in spelling and pronounce it as /k/ (i.e. unaspirated), such as Slavic languages, Northern Germanic languages, Turkish, Dutch, Malay, Hungarian and Finnish.
English and German are notable exceptions to this (using aspiration).
Also, some languages use the Latin letter “w” in spelling and pronounce it as /w/, such as English and Dutch.
German is a notable exception to this, pronouncing “w” as /v/. Also, some languages spell /w/ as “v”.
The IPA symbol /x/ is a bit weird. In Latin, the “x” was pronounced as /ks/, not as /x/. So I think the IPA symbol is more derived from the Greek symbol χ, because very few languages using Latin script would spell “x” and pronounce it as /x/. Listed on Wikipedia are Uzbek, Kurdish and Wolof. Many languages that have the /x/ sound spell it as “ch”, “kh”, “h”, “g” or “r” (as in French “très”).
The IPA symbol /y/ is also a bit weird. Few languages use the Latin letter “y” and pronounce it as /y/. Wikipedia lists Northern Germanic languages, German and Albanian.
Many languages that have the /y/ sound spell it as “u” or “ü”.
Today I learned that some English dialects also have the /y/ sound, for example the last vowel in “few”, /fjy:/.