I more than 80% of the time will fly Long Hauls + Ultra Long Hauls. That way I can get work done while my plane is flying for hours and hours to at the end of it all gain one landing to my stats… so, How do I decide?
Easy! I usually stick to major Bravo airports, and mainly US West Coast (SFO / LAX), Europe (FRA, CDG, ARN, CPH, LHR, etc…) and Asia (HND, ICN, PEK, HKG, SIN, BKK, etc…). That way I usually look for a flight connecting the big major cities of each continent and then fly either a very popular and frequent flown flight among tourists and business i.e. HKG-LAX, or I pick a flight I have never flown before, and there are plenty that I have not flown easily found by randomly pressing on planes currently flying in the air on FlightRadar24.
Sometimes, I search for hidden gems, shorts hauls with heavy aircraft, like Incheon to Da Nang, with Asiana B744 for an example. So a combination od randomly pressing on airborne aircraft on a flight tracking service website and flying popular, densely packed flights, that are known to be crowded and profitable.