Which diagnosis best describes a client who experiences emotional swings from deep sadness to great joy?

The diagnosis that best describes a client who experiences emotional swings from deep sadness to great joy is Cyclothymia. This condition is characterized by a chronic mood disturbance that includes periods of hypomanic symptoms as well as periods of depressive symptoms that do not meet the criteria for a full depressive episode. In Cyclothymia, these mood changes can be less severe than those experienced in bipolar disorders, creating a pattern of emotional fluctuations lasting for at least two years in adults.

In contrast, Bipolar I disorder typically involves one or more manic episodes, which may be preceded or followed by hypomanic or major depressive episodes, and the mood changes are often more extreme and disruptive. Bipolar II disorder is marked by at least one major depressive episode and one hypomanic episode, but it does not include full-blown manic episodes. Seasonal Affective Disorder (SAD) is specifically tied to seasonal changes and is characterized by recurring episodes of depression associated with specific seasons, rather than the more cyclical and continuous fluctuations seen in Cyclothymia.

Therefore, Cyclothymia is the most appropriate diagnosis for a client with the described pattern of emotional swings.