which ocean or sea should be taken as mean sea level measuring an altitude?

Mean sea level is NOT determined from a particular sea or ocean. Don't pay attention to the other answers. They are wrong. 

The geoid is a hypothetical surface from which mean sea level is determined and it redefined periodically. Geodesy is very complicated science that I could not possibly explain in 10,000 character spaces on YA. All I can do is post some hyperlinks which you may or may not have any desire to read. 

https://simple.m.wikipedia.org/wiki/Geoid#:~:text=The%20geoid%20is%20a%20shape,geometrical%20shape%20like%20an%20orange.&text=The%20geoid%20is%20the%20shape,such%20as%20winds%20and%20tides....

https://gisgeography.com/geoid-mean-sea-level/#:~:text=The%20level%20the%20water%20would,surface%20in%20a%20vertical%20datum.

You can start with the word "mean".  Mean means average.  That alone tells you it is not one ocean or one sea.  Now go read CarolOkla.

Meaningless to discuss "mean" sea level if you are choosing a specific sea level.

As a practical matter, no punctual measurement can be used.  That was discovered during extensive land surveying of the 19th and 20th centuries.  Different locations have distinct and unique elevations of sea water, even if you take a median level for that location.  Gravity is not universally identical across the face of the earth, with a level of variance that is measurable in differences in sea level between locations.

As a result, surveys gave conflicting elevations when they relied on different locations as a benchmark for "sea level".  What use is it to declare sea level at a particular location in, say, the Pacific Ocean like perhaps some random beach in Hawaii, when you have no Pacific Ocean to use as a benchmark in England, say?

In effect, we cannot rely upon sea level as an actual benchmark.  Its true elevation relative to the center of the earth is not a constant, and varies by some tens of meters depending upon where on earth you do the measurement.

Any of them. Mean sea level is different from place to place. If you are by an ocean or sea, you need to measure the level, preferably every hour or every day, over a full cycle of 18.6 years.

If you are inland, there are survey methods that can be used to trace MSL back to the nearest ocean. But in modern times, for the past 30 years, we use direct measurements of local gravity.

The medium high one so it will be an average .