There is a sentiment, at least for me personally, that far exceeds the weight and power of not only happiness, but even depths of sorrow and melancholy. It is the feeling of uncertainty, a liminal state where one is caught between happiness and sadness, a state of confusion, of suspension in a melange of feelings that feel foreign. It is the feeling where you are not feeling anything in particular, rather gliding along as you catch vague wafts of certain feelings, a distant blur of happiness there, a hard-to-make-out figure of anger there, and so on until you feel that you are not feeling anything specific at all. Or perhaps that you are experiencing so many discordant feelings at once that they have nullified each other.
This is worse than anything because it is ineffable and apparently sourceless. The strength of happiness need not be mentioned, as you know what it is that is making you happy in most cases, and you know the benefit of being happy. Even sorrow, in spite of its acerbic pain and pervasive sense of lethargy, is feeling something. It is not a fun feeling, it is not wanted, but its genesis can in many instances be traced to one origin, or several. Even if it is untraceable or seemingly without source, you still experience a very palpable emotion and you know that you are sad.
But when you are neither happy nor sad, when you are uncomfortably feeling nothing in particular, there is a dread that takes over your thoughts and your very essence. You cannot name any explicit emotion as the cause of the feelings of strangeness any more than you can name the feelings themselves. It is a daze of ambivalence, an unease, as if in anticipation of something good to happen, or something tragic. It is a spring of potential energy waiting to be converted into unleashed action, and until that happens, the spring creaks with expectant but uncertain tension.
This feeling is one of the worst to me, and I would rather have something I can definitively say is causing me sorrow than to be cast into a pit of nothingness and an inexplicable unease. That is perhaps the only word that can describe it: unease. Restless agitation. An ambiguous disturbance to your internal state. Often temporary and fleeting, but when experienced, it feels so eternal and unchanging.