Best offensive:
GROUND 371
ROCK 340
FIGHTING 319
FIRE 313
ICE 313
WATER 301
FLYING 301
DRAGON 289
DARK 264
PSYCHIC 260
GHOST 260
ELECTRIC 237
STEEL 237
BUG 226
GRASS 205
NORMAL 196
POISON 175
Best defensive:
STEEL 168
ELECTRIC 250
GHOST 250
WATER 252
DRAGON 256
POISON 261
DARK 264
FIGHTING 266
FIRE 267
FLYING 273
NORMAL 274
GROUND 289
BUG 291
PSYCHIC 296
GRASS 309
ROCK 309
ICE 332
Numbers represents total damage given (offensive) and taken (defensive), for currently all 17x17 possible type combinations (includes single types). Calculated by simply adding 0, 0.5, 1, 2, 4 damages.
Does not take in account how common type combinations are, let alone if they exist. Therefore, numbers are theoretical and will differ in practise.
Will try to calculate Best defensive including dual types now.