Dispel Magic says (emphasis mine):
Choose one creature, object, or magical effect within range. Any spell of 3rd level or lower on the target ends. For each spell of 4th level or higher on the target, make an ability check using your spellcasting ability. The DC equals 10 + the spell’s level. On a successful check, the spell ends.
At high level play, I’ve found that players often have long-term buff spells in effect (i.e. Heroes Feast, Mindblank, etc.), which are a major factor in them being able to survive.
If one of these characters is affected by a save or suck kind of spell (i.e. Mental Prison, Irresistable Dance, etc.), the need to remove that spell becomes imperative.
With this in mind, can a caster of Dispel Magic target only a negative spell under the assumption that it is a discrete ‘magical effect’ subject to removal? Or must they target the ‘creature’ and risk removing all spells, both positive and negative?
For reference, we see a potentially unique ‘magical effect’ in this question about Yeenoghu’s flail. This effect is similar to a spell in its function (target is affected by the Confusion spell if they fail the save), but has clear differences from a spell because the spell would require Yeenoghu’s concentration and have a different duration. Conversely, this answer suggests that a ‘magical effect’ only applies to things that wholly separated from the creature.