What is it the most common reason for community acquired pneumonia?
You are right in that there are several ways to split down CAP, and plenty of different pathogens to look at.  But then we are looking for the most common.  Pnumonia is most common in children and adults over the age of 75 (or 65 depending on who's counting).  Luckily there are plenty of good reviews for both populations.
From the pediatric side we have Iroh Tam PY(1):

Pneumonia occurs more often in early childhood than at any other age,
with the exception of adults older than 75 years, and kills more
children than any other disease worldwide

As far as pathogens:

Streptococcus pneumoniae and Haemophilus influenzae type b (Hib) have
been characterized as 2 bacteria predominately responsible for cases
of fatal pneumonia in children.  However, widespread introduction
of Hib and pneumococcal conjugate vaccines has led to significant
declines, especially of Hib, although Streptococcus pneumoniae is
still the predominant bacteria isolated from bacterial CAP in
children.  There is increasing recognition of the prevalence of mixed
bacterial and viral infections, which have been documented in 23% to
33% of cases of pneumonia.

Then we move to the geriatric side of things with Falcone et al (2):

Common pathogens include H. influenzae, enteric gram-negative bacilli,
the atypical organisms, S. aureus, Pseudomonas aeruginosa, and M.
catarrhalis, although the relative prevalence of each tends to vary
among studies. Among elderly individuals with aspiration pneumonia,
gram-negative bacilli are the predominate organisms (49 %), followed
by anaerobic bacteria (16 %) and S. aureus (12 %).

Though I should note, having done them myself, getting accurate cultures is difficult, and viral levels a very likely to be under reported because of handling and skill.

For example, in different prospective studies in which the
microbiologic etiology of CAP was systematically sought in the
elderly, a definite or presumptive pathogen was often reported in &lt;60
% of the patients. (2)

And again in children:

There are 2 main challenges in the diagnosis of CAP: the first is the
definition of CAP, particularly in young children, in whom bacterial
and viral infections can occur with similar frequencies, and in whom
overdiagnosis of mild symptoms and signs may lead to unnecessary
antibiotic use; the second is the identification of a causative pathogen, which is frequently impractical and inadequate in children, and
in whom the failure to isolate an organism can result in unnecessary
antibiotic use. (1)

Taking into account more global sources, I'm actually thinking Hib infection has dropped more due to vaccination than I gave it credit for, putting my bet on S. pneumoniae, and showing my regional experience may not be representative (3, 4, 5, 6).
(1) Iroh Tam PY. Approach to common bacterial infections: community-acquired pneumonia. Pediatr Clin North Am. 2013 Apr;60(2):437-53. doi: 10.1016/j.pcl.2012.12.009. Epub 2013 Jan 12
(2) Falcone M. et al. Pneumonia in frail older patients: an up to date. Intern Emerg Med. 2012 Oct;7(5):415-24. Epub 2012 Jun 12.
(3) Brown JS. Community-acquired pneumonia. Clin Med. 2012 Dec;12(6):538-43.
(4) Kurutepe S, Ecemiş T, Ozgen A, Biçmen C, Celik P, Aktoğu Özkan S, Sürücüoğlu S. [Investigation of bacterial etiology with conventional and multiplex PCR methods in adult patients with community-acquired pneumonia]. Mikrobiyol Bul. 2012 Oct;46(4):523-31.
(5) Tao LL, Hu BJ, He LX, Wei L, Xie HM, Wang BQ, Li HY, Chen XH, Zhou CM, Deng WW. Etiology and antimicrobial resistance of community-acquired pneumonia in adult patients in China. Chin Med J (Engl). 2012 Sep;125(17):2967-72.
(6) Capelastegui A, España PP, Bilbao A, Gamazo J, Medel F, Salgado J, Gorostiaga I, Lopez de Goicoechea MJ, Gorordo I, Esteban C, Altube L, Quintana JM; Poblational Study of Pneumonia (PSoP) Group. Etiology of community-acquired pneumonia in a population-based study: link between etiology and patients characteristics, process-of-care, clinical evolution and outcomes.  BMC Infect Dis. 2012 Jun 12;12:134. doi: 10.1186/1471-2334-12-134.