`Normal (44, 17.5%)
Fire (19, 7.6%)
Water (48, 19.1%)
Electric (17, 6.8%)
Grass (24, 9.6%)
Fighting (11, 4.4%)
Flying (38, 15.1%)
Poison (37, 14.7%)
Ground (24, 9.6%)
Psychic (24, 9.6%)
Ice (10, 4%)
Rock (18, 7.2%)
Bug (22, 8.8%)
Ghost (4, 1.6%)
Dragon (4,1.6%)
Steel (6, 2.4%)
Dark (6, 2.4%)
(The numbers are the amount of that type and the percentage that is of 251.)

Random Factual Tidbits:
113 Pokémon have two types; 138 have but one.
There are no pure Steel, Ice, or Flying types.
(Prior to Gold/Silver, there were no pure Rock or Ghost types, either.)
Flying is the most varied type, having combinations with 14 of the other 16 types.
(Only Fighting and Ghost do not have a Flying type.)

The following Pokémon, due to their type combinations, have a double weakness to the
indicated type(s), and therefore take four times the damage from attacks of those type(s).
Geodude, Graveler, Golem, Onix, Rhyhorn, Rhydon, Larvitar, Pupitar - Water, Grass
Butterfree, Scyther, Ledyba, Ledian, Yanma, Charizard, Moltres, Ho-Oh - Rock
Omanyte, Omastar, Kabuto, Kabutops, Corsola, Wooper, Quagsire - Grass
Dragonite, Hoppip, Skiploom, Jumpluff, Gligar - Ice
Paras, Parasect - Fire, Poison, Flying
Exeggcute, Exeggutor, Celebi - Bug
Gyarados, Mantine - Electric
Sneasel, Tyranitar - Fighting
Magcargo - Water, Ground
Forretress, Scizor - Fire
Heracross - Flying

It has been debated whether Bug / Steel or Dark / Fire is the best combination.
Here are the defenses against types for those combinations. Decide for yourself.

Bug / Steel
400% damage from: Fire
100% damage from: Water, Electric, Fighting, Flying, Rock
50% damage from: Normal, Ice, Psychic, Bug, Ghost, Dragon, Steel, Dark
25% damage from: Grass, Ground
0% damage from: Poison

Dark / Fire
200% damage from: Water, Fighting, Ground, Rock
100% damage from: Normal, Electric, Flying, Poison, Bug, Dragon
50% damage from: Fire, Grass, Ice, Ghost, Steel, Dark
0% damage from: Psychic

Dragon 147. Dratini
148. Dragonair
149. Dragonite / Flying
230. Kingdra / Water
2 pure
1 Water
1 Flying
Ghost 92. Gastly / Poison
93. Haunter / Poison
94. Gengar / Poison
200. Misdreavus
1 pure
3 Poison
Steel 81. Magnemite / Electric
82. Magneton / Electric
205. Forretress / Bug
208. Steelix / Ground
212. Scizor / Bug
227. Skarmory / Flying
2 Electric
1 Flying
1 Ground
2 Bug
Dark 197. Umbreon
198. Murkrow / Flying
215. Sneasel / Ice
228. Houndour / Fire
229. Houndoom / Fire
248. Tyranitar / Rock
1 pure
2 Fire
1 Flying
1 Ice
1 Rock
Ice 87. Dewgong / Water
91. Cloyster / Water
124. Jynx / Psychic
131. Lapras / Water
144. Articuno / Flying
215. Sneasel / Dark
220. Swinub / Ground
221. Piloswine / Ground
225. Delibird / Flying
238. Smoochum / Psychic
3 Water
2 Flying
2 Psychic
2 Ground
1 Dark
Fighting 56. Mankey
57. Primeape
62. Poliwrath / Water
66. Machop
67. Machoke
68. Machamp
106. Hitmonlee
107. Hitmonchan
214. Heracross / Bug
236. Tyrogue
237. Hitmontop
9 pure
1 Water
1 Bug
Electric 25. Pikachu
26. Raichu
81. Magnemite / Steel
82. Magneton / Steel
100. Voltorb
101. Electrode
125. Electabuzz
135. Jolteon
145. Zapdos / Flying
170. Chinchou / Water
171. Lanturn / Water
172. Pichu
179. Mareep
180. Flaaffy
181. Ampharos
239. Elekid
243. Raikou
12 pure
2 Water
1 Flying
2 Steel
Rock 74. Geodude / Ground
75. Graveler / Ground
76. Golem / Ground
95. Onix / Ground
111. Rhyhorn / Ground
112. Rhydon / Ground
138. Omanyte / Water
139. Omastar / Water
140. Kabuto / Water
141. Kabutops / Water
142. Aerodactyl / Flying
185. Sudowoodo
213. Shuckle / Bug
219. Magcargo / Fire
222. Corsola / Water
246. Larvitar / Ground
247. Pupitar / Ground
248. Tyranitar / Dark
1 pure
5 Water
8 Ground
Fire 4. Charmander
5. Charmeleon
6. Charizard / Flying
37. Vulpix
38. Ninetales
58. Growlithe
59. Arcanine
77. Ponyta
78. Rapidash
126. Magmar
136. Flareon
146. Moltres / Flying
155. Cyndaquil
156. Quilava
157. Typhlosion
218. Slugma
219. Magcargo / Rock
244. Entei
250. Ho-Oh / Flying
15 pure
3 Flying
1 Rock
Bug 10. Caterpie
11. Metapod
12. Butterfree / Flying
13. Weedle / Poison
14. Kakuna / Poison
15. Beedrill / Poison
46. Paras / Grass
47. Parasect / Grass
48. Venonat / Poison
49. Venomoth / Poison
123. Scyther / Flying
127. Pinsir
165. Ledyba / Flying
166. Ledian / Flying
167. Spinarak / Poison
168. Ariados / Poison
193. Yanma / Flying
204. Pineco
205. Forretress / Steel
212. Scizor / Steel
213. Shuckle / Rock
214. Heracross / Fighting
4 pure
2 Grass
1 Fighting
5 Flying
7 Poison
1 Rock
2 Steel
Ground 27. Sandshrew
28. Sandslash
31. Nidoqueen / Poison
34. Nidoking / Poison
50. Diglett
51. Dugtrio
74. Geodude / Rock
75. Graveler / Rock
76. Golem / Rock
95. Onix / Rock
104. Cubone
105. Marowak
111. Rhyhorn / Rock
112. Rhydon / Rock
194. Wooper / Water
195. Quagsire / Water
207. Gligar / Flying
208. Steelix / Steel
220. Swinub / Ice
221. Piloswine / Ice
231. Phanpy
232. Donphan
246. Larvitar / Rock
247. Pupitar / Rock
8 pure
2 Water
1 Flying
2 Poison
2 Ice
8 Rock
1 Steel
Grass 1. Bulbasaur / Poison
2. Ivysaur / Poison
3. Venusaur / Poison
43. Oddish / Poison
44. Gloom / Poison
45. Vileplume / Poison
46. Paras / Bug
47. Parasect / Bug
69. Bellsprout / Poison
70. Weepinbell / Poison
71. Victreebel / Poison
102. Exeggcute / Psychic
103. Exeggutor / Psychic
114. Tangela
152. Chikorita
153. Bayleef
154. Meganium
182. Bellossom
187. Hoppip / Flying
188. Skiploom / Flying
189. Jumpluff / Flying
191. Sunkern
192. Sunflora
251. Celebi / Psychic
7 pure
3 Flying
9 Poison
3 Psychic
2 Bug
Psychic 63. Abra
64. Kadabra
65. Alakazam
79. Slowpoke / Water
80. Slowbro / Water
96. Drowzee
97. Hypno
102. Exeggcute / Grass
103. Exeggutor / Grass
121. Starmie / Water
122. Mr. Mime
124. Jynx / Ice
150. Mewtwo
151. Mew
177. Natu / Flying
178. Xatu / Flying
196. Espeon
199. Slowking / Water
201. Unown
202. Wobbuffet
203. Girafarig / Normal
228. Smoochum / Ice
249. Lugia / Flying
251. Celebi / Grass
11 pure
1 Normal
4 Water
3 Grass
3 Flying
2 Ice
Poison 1. Bulbasaur / Grass
2. Ivysaur / Grass
3. Venusaur / Grass
13. Weedle / Bug
14. Kakuna / Bug
15. Beedrill / Bug
23. Ekans
24. Arbok
29. Nidoran (F)
30. Nidorina
31. Nidoqueen / Ground
32. Nidoran (M)
33. Nidorino
34. Nidoking / Ground
41. Zubat / Flying
42. Golbat / Flying
43. Oddish / Grass
44. Gloom / Grass
45. Vileplume / Grass
48. Venonat / Bug
49. Venomoth / Bug
69. Bellsprout / Grass
70. Weepinbell / Grass
71. Victreebel / Grass
72. Tentacool / Water
73. Tentacruel / Water
88. Grimer
89. Muk
92. Gastly / Ghost
93. Haunter / Ghost
94. Gengar / Ghost
109. Koffing
110. Weezing
167. Spinarak / Bug
168. Ariados / Bug
169. Crobat / Flying
211. Qwilfish / Water
10 pure
3 Water
9 Grass
3 Flying
2 Ground
7 Bug
Flying 6. Charizard / Fire
12. Butterfree / Bug
16. Pidgey / Normal
17. Pigeotto / Normal
18. Pidgeot / Normal
21. Spearow / Normal
22. Fearow / Normal
41. Zubat / Poison
42. Golbat / Poison
83. Farfetch'd / Normal
84. Doduo / Normal
85. Dodrio / Normal
123. Scyther / Bug
130. Gyarados / Water
142. Aerodactyl / Rock
144. Articuno / Ice
145. Zapdos / Electric
146. Moltres / Fire
149. Dragonite / Dragon
163. Hoothoot / Normal
164. Noctowl / Normal
165. Ledyba / Bug
166. Ledian / Bug
169. Crobat / Poison
176. Togetic / Normal
177. Natu / Psychic
178. Xatu / Psychic
187. Hoppip / Grass
188. Skiploom / Grass
189. Jumpluff / Grass
193. Yanma / Bug
198. Murkrow / Dark
207. Gligar / Ground
225. Delibird / Ice
226. Mantine / Water
227. Skarmory / Steel
249. Lugia / Psychic
250. Ho-Oh / Fire
11 Normal
3 Fire
2 Water
1 Electric
3 Grass
3 Poison
1 Ground
3 Psychic
2 Ice
1 Rock
5 Bug
1 Dragon
1 Steel
1 Dark
Normal 16. Pidgey / Flying
17. Pidgeotto / Flying
18. Pidgeot / Flying
19. Rattata
20. Raticate
21. Spearow / Flying
22. Fearow / Flying
35. Clefairy
36. Clefable
39. Jigglypuff
40. Wigglytuff
52. Meowth
53. Persian
83. Farfetch'd / Flying
84. Doduo / Flying
85. Dodrio / Flying
108. Lickitung
113. Chansey
115. Kangaskhan
128. Tauros
132. Ditto
133. Eevee
137. Porygon
143. Snorlax
161. Sentret
162. Furret
163. Hoothoot / Flying
164. Noctowl / Flying
173. Cleffa
174. Igglybuff
175. Togepi
176. Togetic / Flying
190. Aipom
203. Girafarig / Psychic
206. Dunsparce
209. Snubbull
210. Granbull
216. Teddiursa
217. Ursaring
233. Porygon2
234. Stantler
235. Smeargle
241. Miltank
242. Blissey
32 pure
11 Flying
1 Psychic
Water 7. Squirtle
8. Wartortle
9. Blastoise
54. Psyduck
55. Golduck
60. Poliwag
61. Poliwhirl
62. Poliwrath / Fighting
72. Tentacool / Poison
73. Tentacruel / Poison
79. Slowpoke / Psychic
80. Slowbro / Psychic
86. Seel
87. Dewgong / Ice
90. Shellder
91. Cloyster / Ice
98. Krabby
99. Kingler
116. Horsea
117. Seadra
118. Goldeen
119. Seaking
120. Staryu
121. Starmie / Psychic
129. Magikarp
130. Gyarados / Flying
131. Lapras / Ice
134. Vaporeon
138. Omanyte / Rock
139. Omastar / Rock
140. Kabuto / Rock
141. Kabutops / Rock
158. Totodile
159. Croconaw
160. Feraligatr
170. Chinchou / Electric
171. Lanturn / Electric
183. Marill
184. Azumarill
186. Politoed
194. Wooper / Ground
195. Quagsire / Ground
199. Slowking / Psychic
211. Qwilfish / Poison
222. Corsola / Rock
226. Mantine / Flying
230. Kingdra / Dragon
245. Suicune
25 pure
2 Electric
1 Fighting
2 Flying
3 Poison
2 Ground
4 Psychic
3 Ice
5 Rock
1 Dragon
1