Air carrier pilot reported they were cleared direct BESTT but fix BESST was entered into the FMS resulting in a course deviation and conflict with another aircraft.

Narrative

We were conducting Flight XXXX ZZZ to CLT. We were climbing through 14600 feet somewhere around MEI when Center told us to go direct BESTT. The First Officer (FO) was the pilot flying and the FO had input the flight plan. Both the Captain and the FO failed to catch on the initial flight plan verification that the FO had input 'BESST' instead of 'BESTT'. When the FO input the 'direct to' in the FMS; the plane turned to the wrong fix. It turned to BESST instead of BESTT. This meant that the airplane started turning off course to the left. According the FlightAware which I checked after the flight; we got as far as 20 degrees off track to the left. The proper track would have been about 72 degrees toward BESTT.Instead; the plane ended up on a 52 degree track toward BESST which is up near the Great Lakes. I didn't catch the error until ATC called us with a traffic alert and told us to immediately turn back to a heading of 100 degrees to get us back on track toward the correct fix. According to ATC; there was a traffic conflict; I'm not aware of how close we got to the other traffic. This occurred on Memphis Center frequency of 125.97. After we entered the proper fix in the FMS; we continued to CLT without further incident.Cause: Failure to check spelling of all fixes in the FMS; and also failure to check total mileage comparison between FMS and filed flight plan which may have also trapped the error. Also; the FO told me that he had manually typed the fix in the FMS instead of selecting the BESTT transition from the dropdown list in the JONZE arrival. I explained to the FO that he should select the transition from the dropdown list instead of typing it manually.Suggestion: Carefully check spelling of all fixes while still at the gate before takeoff....also compare mileage between FMS flight plan and flight plan on the release.