Had sex with my ex 5 days before my ovulation then with my boyfriend the day before and the day of. Who most likely is the father?

The only sure fire method to determined paternity is a DNA test f=after the birth.
Dates are far to close for any other method to be reliable.

There is no way to guess, either can be equally.

Question on how you know ovulation. A online chart, may or may not be correct, and it is only an estimate. So unless you are testing ovulation medically that is only an estimate.

If the sex was two perhaps 3 week apart, a guess would be easier. But this way, a DNA test after birth will be needed

Since some sperm may survive for several days before ovulation, and the encounters were just a few days apart, either one could be the father.

In fact, if you were pregnant with twins, each twin could have a different father.

As was said, a DNA test will be necessary.

There is no most likely, only a test after birth can tell who the father is.

Tell them both because you have to have a DNA test done to find out who the lucky man is.