Who is the longest surviving characters in Game of Thrones series?

If you mean longest living as in the one who has lived the longest physical years, the answer is probably Aemon.

If you mean who has survived the killing that have taken place throughout the books, then there would be several runner ups that would share first place as they have been alive since the first book.

Character age chart.

Another age chart.

Proves Aemon is older than a lot of characters. Proves he his 102.

From the wikia:

Around one hundred years old, Aemon is thought to be one of the oldest
living men in Westeros, though he has retained all of his wits and his
mind is still very sharp.

Aemon unless you count wights, or giants or even the cold walking people,is the oldest at 102 years of age.

Canon proof: That is the were the wikia say they got their information from.