The bulkiest nitrogenous base out of the four main ones (Adenine, Guanine, Cytosine, Thymine) is Guanine.

Here's why:

* Guanine has a two-ring structure (purine) and a complex side chain, making it the largest and most bulky.

* Adenine is also a purine but has a simpler side chain than Guanine.

* Cytosine and Thymine are both pyrimidines, having a single-ring structure, making them smaller than purines.

Therefore, Guanine takes the crown for the bulkiest nitrogenous base.