For number ten I weigh in with yet another film I've reviewed in more detail on my film-review blog: KING KONG VS. GODZILLA.
Without repeating the points I make in my review, I'll point out that even though this is not one of the best-scripted crossovers, it sustains historical importance as the film which revived Godzilla's career.
Consider: the original 1954 GODZILLA was a smash in Japan, and the Americanized version-- the one that circulated both in the U.S. and in most other countries in the world-- was dominantly a box-office success as well. However, Toho Studio's quickie 1955 follow-up GODZILLA RAIDS AGAIN underperformed at the Japanese box office. That film was not released in the U.S. for another four years-- twice the time it took for the original to receive a U.S. release-- and the American producers renamed the titular monster "Gigantis." This heavily recut version failed as well, so that for several years Godzilla remained in mothballs.
Ironically, KING KONG VS. GODZILLA-- which revived Godzilla's marquee appeal-- was not originally projected as a Godzilla film, though it too was intended as a crossover of sorts. Animator Willis O'Brien, the chief animator of 1933's KING KONG, attempted to sell various studios on a project initially entitled "King Kong vs. Frankenstein." As this Wikipedia essay explains in more detail, the O'Brien project morphed into KING KONG VS. GODZILLA.
Though O'Brien never took part in this project, his brainchild Kong gets more sympathetic treatment than that nasty reptile Godzilla. The Big G is in some ways more powerful than Kong, who has no counter-force against Godzilla's atomic breath. Still, Godzilla here lacks the apocalyptic menace he has here, just as he did in GODZILLA RAIDS AGAIN. Not until the fourth film in the series, wherein Godzilla faced off against Toho's newly minted monster-star Mothra did Godzilla begin to seem like a force of nature again-- and even then, this fearsome persona would often be modified by dollops of trivial comedy.
The most important aspect of KING KONG VS. GODZILLA-- aside from its significance in reviving the "Big G" franchise-- was the filmmakers' decision to build an entire film around the conflict of two giant creatures. Toho's first monster-vs.-monster combat appears in GODZILLA RAIDS AGAIN, wherein the King of the Monsters meets his first foe of this type, the quadruped Anguirus. However, said combat was not the main focus of the narrative, and I've speculated that it may have been included for two possible reasons: (1) in blatant imitation of the monster/monster fights in the 1933 KING KONG, and (2) as a means of cutting down on some of the special-FX. KING KONG VS. GODZILLA-- which even uses the same visual trope seen in RAIDS, where two giant monsters knock down a building as they battle-- made the big fight the central selling-point. After that, most Godzilla films took note and made sure to give the starring a monster one or more gargantuan opponents-- resulting in yet more crossovers, though in this list I'll only address one more, which will stand for all of the Toho Godzilla productions.