General:
* Genuine: This is a very close synonym, emphasizing authenticity and realness.
* Honest: This emphasizes truthfulness and lack of deception.
* Truthful: This emphasizes accuracy and lack of falsehood.
* Authentic: This emphasizes originality and genuineness.
* Real: This emphasizes the lack of pretense or artificiality.
* Heartfelt: This emphasizes deep emotion and feeling.
More specific:
* Candid: This implies openness and frankness.
* Forthright: This implies directness and straightforwardness.
* Unfeigned: This emphasizes the lack of pretense or insincerity.
* Earnest: This implies a seriousness of purpose.
* Wholehearted: This emphasizes complete commitment and dedication.
More formal:
* Veracious: This implies truthfulness and accuracy.
* Upright: This emphasizes moral integrity and honesty.
The best synonym for "sincere" will depend on the specific context and nuance you want to convey.