Key ingredients for classic Sauce Bolognese (Ragù Bolognese)?

@Stefano's answer is obviously the accepted one, linking to the thing closest to being an official recipe. I still want to add an alternative answer though, that includes some empirical evidence. It combines the results from recipes highly ranked by Google with the ingredients from that official recipe.

After gathering the recipes (and assigning the "official" source double score), I've ranked the ingredients based on appearance. Here are my findings.

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Must Haves:

• Onion
• Carrot
• Celery
• Ground Beef
• Tomato Paste
• Salt
• Pepper

Should Haves:

• Olive oil (usually extra-virgin, but the "official" source mentions regular oil)
• Pancetta
• Milk
• Ground Veal and/or Pork ^†
• Dry White Wine (incidentally recipes mention red wine instead)
• Beef Stock (incidentally chicken stock instead)

^{† The only ingredient not in the "official" source's recipe, that is found in most other recipes.}

Honerable mentions:

• Cream (the only "official" ingredient not found in most recipes)

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Ingredients usually not mentioned:

• Bacon (instead of pancetta)
• Tin crushed tomatoes
• Fresh tomatoes (never mentioned!)
• Sieved tomatoes
• Butter
• Cloves
• Bay Leaves
• Nutmeg
• Cinnamon
• Basil
• Oregano
• Parsely
• Sugar
• Garlic

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You can easily check my findings in this document. A summary of the scores for all ingredients is as follows:

Ingredient Grouping     Ingredient                    Summed Score
----------------------  ----------------------------  --------------
Onions                  Onions                        9
Carrot                  Carrot                        9
Celery                  Celery                        9
Ground Meat             Ground Beef                   8
Tomatoes (processed)    Tomato Paste                  8
Salt                    Salt                          8
Pepper                  Pepper                        8
Bacon/Pancetta          Pancetta                      7
Dairy                   Milk                          7
Ground Meat             Ground Veal                   5
Wine                    White Wine (dry)              5
Stock                   Stock (beef)                  5
Ground Meat             Ground Pork                   4
Olive Oil               Extra-virgin Olive Oil        4
Olive Oil               Regular Olive Oil             3
Dairy                   Cream                         3
Wine                    Red Wine                      2
Stock                   Stock (chicken)               2
Garlic                  Garlic                        2
Bacon/Pancetta          Bacon                         1
Tomatoes (processed)    Tomatoes: Sieved              1
Tomatoes                Tomatoes (tin crushed)        1
Wine                    White or Red (nonspecified)   1
Butter                  Butter                        1
Cloves                  Cloves                        1
Bay Leaves              Bay Leaves                    1
Nutmeg                  Nutmeg                        1
Cinnamon                Cinnamon                      1
Tomatoes                Tomatoes (fresh)              0
Stock                   Stock (veal)                  0
Basil                   Basil                         0
Oregano                 Oregano                       0
Parseley                Parseley                      0
Sugar                   Sugar                         0

If we forget for a second about the "official" codified recipe that has already been mentioned, we can try to think about the philosophy behind Bolognese sauce and this might lead us to the answer what are the key ingredients for this sauce. First of all this is something that you eat very often so it needs products that are readily available all year round and are more or less cheap.

Now what are the two most important ingredients of Italian cuisine - my opinion is that they are wheat and tomatoes. Wheat we've got already in the pasta so we're left with the tomatoes. Tomato paste is something that all Italian families prepare in the summer and have a huge stash in the pantry to last them for the entire winter. Another important part of the Italian cuisine is the so-called Soffritto - chopped onions, carrots and celery, used as a base for many stocks, sauces, soups and stews, similar to the french mirpoix.

This for me would be the second most important ingredient (or three ingredients to be pricise) as it's available all year round and cheap. The third ingredient would be the meat - beef or veal, it doesn't really matter for me, even if there's some pork in the mix, I'd still call it a Bolognese. The 4th ingredient would be extra virgin olive oil, cause we can't cook our veg without it and it adds flavor too.

And if I can pick one other ingredient that would be the wine, as it's also an important part of Italian culture and way of life. And I would put white wine in the summer and red wine in the winter. Now with these five ingredients only you can make a great Bolognese sauce, it will not be the best as we're lacking the pancietta or the beef stock, but still it will be very good and true to the original.