"less than" 小于, better than "least or at least" 至少.

I think "less than" is correct. I also agree with Peachey on using "less than the others" or "less than them.

Was I least than the rest doesn't work, at least should add a 'THE' before least. But still incorrect, when you use a superlative word you don't need a 'THAN' anymore. So less is better. And 'but' here seems no good, perhaps it's except. On the contrary, but usually is not after an 'ALL' but a 'NO' like that funny Korean pop song: Nobody nobody but you.

"It was as if my dad loved all his children, but me. I wondered why. Am I the least among the rest?" "least" here functions as an adverb which could mean the smallest or lowest degree below all others. I think that's how you say it... :)

"It was as if my dad loved all his children, but me. I wondered why. Was I less than the rest?" is more correct. Perhaps use, "less than the others" or "less than them"? "Least" is something specific. If you wanted to use "least", it would be: "It was as if my dad loved all his children, but me. I wondered why. Was I the least of them?"