2. In my opinion...
In my opinion, Curitiba is the coldest city in Brazil.
I think Porto Alegre is the coldest city in Brazil.
3. Rules
ADD – THE + EST = COLD - THE COLDEST
SMALL – THE SMALLEST
ADD – THE MOST + ADJECTIVE = POPULAR – THE MOST POPULAR
ADD – THE + IEST = PRETTY = THE PRETTIEST
IRREGULAR = GOOD = THE BEST