Adjective that describes "boosting one's ego"?

             "Being one of a kind is gratifying, but lonely at times."

gratify verb; 3rd person present: gratifies; past tense: gratified; past participle: gratified; gerund or present participle: gratifying

give (someone) pleasure or satisfaction. "I was gratified to see the coverage in May's issue"

synonyms: please, gladden, make happy, delight, make someone feel good, satisfy; see Google gratify

I'd suggest "flattering".

flattering (adj) "pleasing or gratifying the vanity of"

• "What really flatters a man is that you think him worth flattering" (George Bernard Shaw).
• "It was flattering to be told how indispensable he was." TFD

Probably no less formal then “uplifting,” but “laudable” would perhaps also work, as would any of its synonyms, such as "commendable," “praiseworthy,” or “prideworthy.”

(Just an afterthought, but to the extent that the above suggestions are still a bit too formal, the ‘simple’ word “great” [or just “good,” depending on how high you want to boost the ego] could work nicely, especially in your specific example to emphasize the frustrating contrast with the fairly well-known “it's/that’s great, but …” construction.)

Ego and pride are somewhat similar, sometimes conflated. Based on that, I looked for a word which means to boost one's pride. Depending on the actual idea you're trying to express, one of these might be helpful:

Being one of a kind _ _, but lonely at times.

is exalting/exalts

is glorifying/glorifies is aggrandizing is dignifying is ennobling is an honour

Also, this site might be helpful: http://www.thesaurus.com/browse/exalting