Libya is probably, structurally, the easiest.
Sudan has a lot of problems, and given that there is already a major divide between Upper Egypt and Lower Egypt (with regards to Lower Egypt being more developed and affluent with Upper Egypt generally seen as little more than a land of farmers and migrant laborers), throwing Sudan into the mix likely wouldn't do any great favors.
Libya is small, and depending on the timeframe has either already discovered its oil or it's about to. The problem of course would likely be a disparity in influence and population. Egypt is the wealthier, more populous, and all-around superior partner in a union between Egypt and Libya. It's not at all hard to imagine a situation where Libyan oil wealth is going into Egyptian pockets with very few returns for the Libyans themselves.
That said, out of the Egypt-Sudan union, which poses all kinds of issues for water use and wealth disparity, Egypt-Syria which is divided firstly by geography and later by culture and demographics, Egypt-Libya remains the most viable option, and even then it will take careful management to keep Cairo from being too much the superior to Tripoli.