Best for mostRobin Campbell · 1969
For most readers this is the best way to meet Seneca — even though it's a selection, not the whole. Campbell picks about 40 of the 124 letters and translates them with real wit, catching the tight, quotable style. You get the essential Seneca without the slog, for around $14. If you later want all of it, you'll know where to go. This is where I'd start.
Most completeMargaret Graver & A. A. Long · 2015
The scholar's complete Seneca: all 124 letters in a clear modern translation, each with a thematic summary and serious notes. The one to own if you want everything, or you're studying him. More money and more book than most first-timers need — but it's the standard scholarly edition.
Most beautifulRobin Campbell · 1969
The same Campbell selection in Penguin's cloth-bound hardcover, with Coralie Bickford-Smith's cover. It's a popular gift for exactly that reason — the best-loved short Seneca, made into a handsome object. You're paying for the binding, not for more text.
SenecaLetters from a Stoictr. Richard M. Gummere
The free classicRichard M. Gummere · 1917
Richard Gummere's early-1900s translation is out of copyright, and it's complete — all 124 letters, free online. The English is a century old and more formal than Campbell's, but if you want every letter without spending anything, this is the option no one mentions. Faithful, complete, free.