The superlative of sincere

Solution 1:

Consider the common phrase sincerest form of flattery. Where I'm from I hear sincerest more often than most sincere, though it's close. I never hear sincerer. Google Ngrams trends the same as my personal experience, and it looks like there was a shift from most sincere to sincerest in the 1980s but the difference is negligible regardless:

Solution 2:

Perhaps more appropriate Ngrams (English being so unpredictable) are these.

These suggest that (and my opinion is in agreement, but more data would be more persuasive):

1) 'Sincerest' is preferred to 'most sincere', though both are in use.

2) The degree of preference is dependent on the following noun group. With '... form of', the periphrastic alternative is much less frequently used.

Solution 3:

"My most sincere gratitude" would be the most common form used. Superlative for long words tend to be made using more or most instead of the suffixes -er -est.