I always though Isaka just used other memories before he got Weather. He wanted to strongest memory so he just threw away the weaker ones he had every time he got a better one.
Its more like the grade and "refinement" (as Isaka stated once) of each memory.
Lets take a look. The highest and most powerful memory will be graded A
S Rank: Cyclone, Heat, Luna, Fang, Joker, Metal, Trigger, Xtreme, Prism, Accel, Trial, Skull
->These memories are pure and do not have corruptive or addictive features in the un-pure memories, they are not able to transform the human body into a supernatural being without the power of a specific driver.
A Rank: Terror, Clay Doll, Nazca, Taboo, Smilodon
->These memories have gold plating and are only given to high ranking officers of the Museum
B Rank: Weather
->Memory(ies) that are stronger than C and D rank memories due to the refinement but requires great control and knowledge of the memory (like nobody could use weather like Isaka).
C Rank: Everything else.
->Unique memories that are sold to the public; basically every other memory.
D Rank: Masquerade, Anamanocalis (or however you spell it)
->Mass production memories. Low on power and basically anyone can use it.
T2 Rank: A-Z
->26 unique pure memories that are stronger than S Ranking memories. Without the movie, not alot can be said, but we do know that it does not corrupt minds, and can be slotted into certain gears (ie. Bomb memory). But all confirmed destroyed.
(This is not official infomation)
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My Theory.
The 7 S Ranks (all of Double's memories) were made by Fumine/Shroud, but when Ryubee takes Raito/Philip away, she goes into hiding along with the incomplete double driver (aka the lost driver). She seeks help from a certain detective inorder to save her son. She gives him the lost driver and the skull memory for him to use (along with the double driver and 6 memories to use after Raito is saved). With a strong will, Shroud knew that he would not be overpowered by the memory. But when Skull dies in Begin's Night, Shroud concludes that kindness will not save her son (even though he is safe with shotaro as Double). She creates the power-up for double Xtreme. As she watches Shotaro and Raito fight dopants, she feels the Shotaro is holding back Raito in is growth of power, and seeks someone who will take out Ryubee instead. She comes across Isaka Shinkro, a corrupted doctor hell bent on taking out Terror. To assist in his greed, Shroud gives Isaka a B-Rank weather memory. Isaka could not be Raito's true partner because he did not have a unique philology to avoid Terror's mental strikes. Isaka mastered the weather memory quickly and started murdering people. Along with the shock of the monster she created, she discovered the sole survivor of the Terui family, Ryu. She learned that he has the unique philology that she has been looking for, and takes advantage of his hate (thus why she said i am one who supports your revenge). To train and test him, he gives him the Accel memory, Engine memory, Engine Blade, and the Accel Driver. As she watches Ryu grow into the machine she wants him to be, Isaka exceeds her calculations and becomes a beast. Due to Shotaro's idioticness, Raito almost gets killed by Isaka (during the Nightmare episodes), and saved him with the Xtreme Memory. She intended for Ryu to achieve xtreme with Raito, but Shotaro exceeded her expectations and become CycloneJokerXtreme instead of CycloneAccelXtreme. Shroud became disappointed and created the Trial memory in order to shape Ryu even more to become the ultimate double, but he had already begun losing his hatred. Ryu demanded for more power to early and Shroud played along and tested him (and he failed). All ends well with Accel Trial taking out Weather. But Shroud need Weather in order to defeat Terror. With him out of the picture, she relies on the riders. He gives some random guy a unique memory, Old. Old worked similarly to the Terror memory, and showed that Shotaro was not capable of fighting Terror. But with Accel's resolve, Shroud gives up in trying to create a Double of hate, and leaves the rest to the Riders.[/hide]