General positive opposites:
* Cheerful
* Joyful
* Happy
* Bright
* Lively
* Vibrant
More specific opposites depending on context:
* Hopeful (if dismal refers to a lack of hope)
* Prosperous (if dismal refers to economic hardship)
* Successful (if dismal refers to a lack of success)
* Splendid (if dismal refers to a lack of beauty or grandeur)
The best antonym will depend on the specific context in which you're using the word "dismal."