Best for mostRobin Buss · 1996
This is the one to get, and it isn't close. Buss's 1996 translation is complete and unabridged — which matters more here than with almost any other classic, because the cheap Monte Cristos are often quietly cut, and the old Victorian translations softened the parts the censors didn't like. Buss is lively, modern, and gives you the whole thing. This is where I'd start.
Best for going deepDavid Coward · 2008
Oxford's complete translation by David Coward, a prize-winning Dumas translator, with the fuller scholarly apparatus — introduction, notes, a chronology. Also unabridged. If you want the same complete novel with more context around it, this is the alternative to Buss.
Most beautifulRobin Buss · 1996
The same complete Buss translation, in Penguin's cloth-bound hardcover with Coralie Bickford-Smith's cover. It costs more and it's heavy — 1,200-odd pages in hardcover is a commitment to hold. But as an object to own and keep, it's the nicest version of the right translation.
Alexandre DumasThe Count of Monte Cristotr. Anonymous (1846)
The free classicAnonymous (1846) · 1846
The original 1846 English translation is out of copyright, so it's free online and turns up in the cheapest print editions. It's complete, but the English is Victorian and a few of the racier passages were softened. Perfectly readable, and free — just know it isn't the full-flavored modern Buss.
The short versionLowell Bair (abridged) · 1956
A warning more than a recommendation. If you find a Monte Cristo that's only 500-some pages, it has been abridged — sometimes cut by a third. This is the most common one. Fine if you want the plot fast, but you're leaving a lot of the book on the table. I'd avoid it unless length is the dealbreaker.