What are some best Text Representation techniques in NLP
There's no simple answer to this question. As far as I know in general the choice depends mostly on the type of classification:


Bag of Words (usually with tf-idf weights) is a simple but quite efficient representation for classification based on the text topic or similar, assuming the classes are reasonably distinct from each other.
Word embeddings are a more advanced option for semantic-based classification. They can handle more subtle semantic relations but require being trained on a large training corpus. Using pre-defined embeddings can be a solution but then there's the risk that the original training data isn't perfectly suitable for the dataset.
N-grams models can be used in many different ways but are often chosen when the classification involves syntax and/or writing style. Note that the higher the value $n$, the larger the training corpus needs to be, this can also be taken into account in the choice.



  I might have around 40 categories and then around a same number of sub-categories upto 4 levels.


It depends on the data but 40 classes is already a very challenging classification task. For the sake of simplicity let's assume a uniform distribution over classes: a random baseline accuracy would be 1/40 =  2.5%. Of course it depends on the data and a good classifier will do better than that, but don't expect too much... 

Now 4 levels of 40 sub-categories means 40^4 = 2.5 millions classes! Even assuming you have enough data (say around 10 instances by class in average, that is 25 millions instances!), it's very unlikely that a classifier will be able to predict anything useful from such a huge amount of classes.