It's the meaning behind the words that make them offensive.
Wanker, twat and pratt all indicate the recipient is intellectually challenged, but each word is usually reserved for a particular type of idiocy and is often preceded by the word stupid.
sod and bugger are usually reserved for the children, or the childish. An adult would be a stupid one, a child a little one.
The 'C' word, which is censored here, is the worst insult you can call anyone. It means they are utter scum and a bit more besides.