lateral thinking - A Single Letter is missing from each, can you figure it out to find the answer and the question?

Grasshopper, in order to know the answer you must first figure out the question.

H, H, D, P, ?                     (2)
P, L, T, Q, P, H, H, ?            (5)
R, C, C, U, B, A, ?               (9)
A, K, Q, J, T, N, E, ?           (11)
M, V, E, M, J, S, ?              (14)
D, P, N, G, C, M, ?              (15)
M, K, S, P, S, ?                 (16)
Y, C, A, G, ?                    (18)

A, T, G, C, L, V, ?              (48)
Z, O, T, T, F, F, S, S, E, N, ? (112)  
H, H, L, B, B, C, ?             (613)
A, A, N, S, A, ?             (371017)

Hint 1:

The answer is 3 letters long

Hint 2 (Bounty Hint):

H, H, D, P, ?, Z.                     (2)  
M, K, S, PH, S, ?, DP, DD, DQ...     (16)

Also anyone who has a better title LMK

Answer

Alright, I'm going to start a partial answer, we'll see if it goes anywhere.

Missing letters:

Edit: Thanks to minitech for number 6! Thanks to Len for number 1! Thanks to Geobits for number 7!

1. H (Hestia, Hades, Demeter, Poseidon, Hera) - Cronus and Rhea's Children

2. O (Point, Line, Triangle, ..., Heptagon, Octagon) - Polygon by number of sides
3. I (Russia, Canada, China, United States, Brazil, Australia, India) - Countries by Area
4. S (Ace, King, Queen, Jack, ..., Eight, Seven) - Cards
5. U (Mercury, Venus, Earth, ..., Saturn, Uranus) - Planets
6. M (Delaware, Pennsylvania, New Jersey, ..., Massachusetts, Maryland) - US States by Ratification Date
7. B (McDonalds, KFC, Subway, Pizza Hut, Starbucks, Burger King) - Largest Fast Food Chains in the World by number of stores
8. R (Yankees, Cardinals, Athletics, Giants, Red Sox) - Most World Series titles
9. L (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra) - Signs of the zodiac
10. T (Zero, One, Two, Three, ..., Nine, Ten) - Counting to ten
11. N (Hydrogen, Helium, Lithium, ..., Carbon, Nitrogen) - Periodic Table Elements

12. E (Asia, Africa, North America, South America, Antarctica, Europe) - Continents by Area

Cleaning up the list and adding the numbers gives us:

H 2
O 5
I 9
S 11
U 14
M 15

B 16
R 18
L 48
T 112
N 613
E 371017

EDIT: From Deusovi's answer below

"Deciphered the numbers based off VictorHenry's answer! All I had to do was... add a couple suggestive spaces:"

H 2
O 5
I 9
S 11
U 14
M 15
B 16
R 18
L 4 8
T 1 12

N 6 13
E 3 7 10 17

Each set of numbers tells us where that letter is located in the final phrase:
1 2 3 4 5 6 7 8 9101112 131415161718
T H E L O N E L I E S T N U M B E R
So the final answer is ONE!