Ultimately it will be campaign-dependent; ask your GM for guidance
I suggest that you speak with your GM about what language options will actually be useful (or even available) for the campaign. You can either choose a useful language from that or you could choose a language you want and then work with the GM to make that language useful in the campaign.
Most GMs I have had have detailed what languages will be prevalent in their campaigns to prevent the PCs from choosing a language only for it to be utterly useless all campaign long and the ones that did not do this helped to make my various character and background decisions matter in the campaign, be those languages, skills, or something else entirely.
Here are some useless totals
DnD Beyond allows you to search through monsters and one of the search settings there (under "show advanced filters") is to sort by languages; using this we can conclude the following (note that 29 monsters know all languages and are included in each one of these totals):
- 222 know Draconic
- 164 creatures know Abyssal
- 147 know Undercommon
- 133 know Infernal
- 75 know Sylvan
- 73 know Deep Speech
- 42 creatures know Celestial
- 51 know Primordial which has four mutually intelligible dialects:
- 64 know Terran
- 53 know Ignan
- 57 know Auran
- 73 know Aquan
Note that this search is not perfect, in fact, far from it; many creatures have variants that are counted as separate monsters (dragons have multiple colors/metals, demons have summoner variants...), nothing is said about the distribution of these monsters (their average CR, or anything similar), overlapping languages are unaddressed (monsters that know multiple languages, especially including common), and a given language's chance of making an appearance in a campaign is known only to your GM.