This would be the shortest and most succinct Norwegian term of praise for 'the most wonderful mother in the world'. However, please note the importance of keeping 'beste' and 'mor' in two separate words. The term 'bestemor' (in one word) means grandmother.

If you'd prefer a more embroidered version, the term above could also be traslated as follows:
Den beste moren i hele verden.

Hope this will help you out!