I won't include a poll as there could be many, many options.
Just consider that many players over-play or play the wrong event, whether it be on the wrong surface or at the wrong level (ATP, challengers, futures).
A common theme is for players to play too many ATP events when they're out of form, hence losing lots of confidence and risking catastrophic dips in the rankings. Conversely, when in-form, some players don't utilise it and play too many challengers, when they could do damage on the main tour and potentially win more points and money.
Who could achieve better results if they scheduled their seasons a bit better?
Not playing at ATP level consistently and actually far from it now but I nominate Federico Delbonis, for sure. Takes huge breaks between tournaments at Challenger level many times a year, sometimes plays 3 or 4 tournaments in a row and maybe gets a couple of early exits which hurt him and then comes the long break when he could've played some of the weakest ATP events in between or even continue the run at Challengers considering a lot of the times there's no fatigue factor (at least from actual on court competition). I guess, however, money is probably an issue in his case so wouldn't criticize too much.