I typically like buying sealed items, so price and rarity are what make mine hard to complete.
Figure - Twistoid (not paying $200-$300 for this guy, so it's a money thing)
Vehicle - Attak Trak
Playset - Fright Zone (finally got one about 3 years ago)
I'll add a beast and accessory column:
Beast - Turbodactyl
Accessories - Scubattack