-- Database schema
| Question : questiontext [ TEXT ] , questionid [ INTEGER ] primary_key | Survey : SurveyID [ INTEGER ] primary_key , Description [ TEXT ] | Answer : AnswerText [ TEXT ] , SurveyID [ INTEGER ] , UserID [ INTEGER ] , QuestionID [ INTEGER ] |
-- -- Betweeen 2016 to 2019, which year recorded the highest number of people with mental illness?

SELECT T1.SurveyID FROM Answer AS T1 INNER JOIN Question AS T2 ON T1.QuestionID = T2.questionid WHERE T1.SurveyID BETWEEN 2016 AND 2019 AND T2.questionid = 34 AND T1.AnswerText LIKE 'Yes' GROUP BY T1.SurveyID ORDER BY COUNT(T1.UserID) DESC LIMIT 1;