#1 Marvel.

Confirmation that most Marvel characters share a world by default in Marvel Mystery Comics#8 when Namor and the android Human Torch first met and fought.

Marvel to DC - blended world in Superman vs Amazing Spider-Man in 1976. Note that this story has been ratified as being a legit alternate reality both by the very first issue of What If?, and by flashbacks in Avengers Forever.
    Sneaky unofficial crossover/near cameos (temporary reality merger?) Heroes from both companies attend the Rutland Halloween Parade, and events from Amazing Adventures#16 leads into events in Justice League of America#103 which then led into Thor#207.
    Sneaky unofficial crossover/cameos (temporary reality merger?) - Aquaman (his hand, at least) appears in Sub-Mariner I#72
    Reality jump (unofficial cameo) - Mantis crosses over to DC's Earth-1 to turn up as Willow in Justice League of America#142
    Unofficial cameos - first Marvel character in DC in ??; first DC character in Marvel in ??
    Reality crossover in DC vs Marvel in 1996. 
    
Marvel to IPC/Fleetway - blended world in Pow!#33 (1967) - Nick Fury meets Dare-a-Day Davy

Marvel to Godzillaverse - incorporated Godzilla into 616 in Godzilla#1/connection confirmed in #3 when Godzilla battled Champions of LA.

Marvel to Conan/Hyperborea - incorporated Conan into 616 in Conan the Barbarian#1/connection confirmed in Marvel Team-Up I#79 when Conan character Red Sonja and Conan foe Kulan Gath encountered Spider-Man in modern day NYC.

Marvel to Doc Savage - incorporated Doc Savage into 616 in Doc Savage#1/connection confirmed in Giant Size Spider-Man#3 when Spider-Man fought foe previously battled decades earlier by Doc, strengthened in Marvel Two-in-One#21 when Doc and his allies met the Thing and Human Torch.

Marvel to Tarzan - incorporated Tarzan into 616 via his foe Abdul Alhazred (Tarzan#15) later fighting Wolverine (MCP I#152). Alhazred was mentioned earlier (MCP I#62) but not seen until #152, where the visuals were seen to match the Tarzan version.

Marvel to Cthulhu mythos - incorporated into 616 via Abdul Alhazred, a Cthulhu Mythos character, as listed above. Marvel has also adapted some mythos stories, but I'm not sure if any of them crossed over with Marvel characters earlier than this, apart from a few that Conan had run ins with.

Marvel to Solomon Kane - incorporated Kane into 616 via his first appearance in Monsters Unleashed#1/confirmed when he encountered Marvel's Dracula in Dracula Lives#3.

Marvel to Doctor Who - confirmed as sharing multiverse when Doctor encountered Merlin in Doctor Who Monthly#60, and then this version of Merlin was shown to be the one known in 616 too in The Daredevils#1. Further ratified when the Doctor visited Earth-616 to dump Death's Head there in Death's Head I#8.

Marvel to Survival Geeks - The Survival Geeks strip in 2000AD#2177 includes groups of multiversal Survival Geeks, including a group who are clearly versions of the Fantastic Four.

Marvel to Star Trek - reality jump link from when X-Men encountered the original Trek's Enterprise crew in Star Trek/X-Men.

Marvel to Ghostbusters - slightly tenuous, but Combat Colin, a Marvel UK humour character who appears to be in 616 via various encounters with the likes of Doctor Doom, also encountered aliens known as the Gwanzulum in Transformers and Action Force#160; the Gwanzulum then faced off against the "Real" Ghostbusters in Real Ghostbusters#9.

Marvel to Thundercats - see above; the Gwanzulum took on the Thundercats in Thundercats#66.

Marvel to TMNT - blended reality in Comic Relief Comic.

Marvel to Dan Dare - cameo of Dare's foe Mekon in The Daredevils#6; blended reality in Comic Relief Comic

Marvel to Time Spirits - reality (time) jumping - Varnae the Vampire, foe of both Conan and Doctor Strange, encountered Cusick in Time Spirits#4

Marvel to Elric - reality jumping - Marvel's Conan encountered a reality jumping Elric in Conan the Barbarian I#14

Marvel to Planeta (Spanish comic company) - counterparts - members of both Iberia Inc and Sirio-10 of Anibal Gris were seen among the multiversal Avengers summoned to battle one another in Avengers Forever#11

Marvel to Grimjack - probably reality jumping, might be counterpart/unauthorized cameo - Professor X and Lilandra met with John Gaunt (a.k.a. Grimjack), going by the name Haggard, in New Mutants# I#50. Important, as Grimjack then links them to many other First Comics characters

Marvel to Nexus - probably reality jumping, might be counterpart - Nexus is also present in an unauthorized cameo in New Mutants I#50

Marvel to Herculoids - probably reality jumping, might be counterpart - the Herculoids are similarly present in an unauthorized cameo in New Mutants I#50, thus also providing links to other Hanna Barbera superheroes

Marvel to Popeye - unauthorized cameo - Popeye attended Lilandra's state ball on the Shi'ar Throneworld in X-Men I#125. It might be interesting to note that DC's Phantom Stranger is also in attendance. 

Marvel to Archie - counterparts? - official crossover in Archie Meets Punisher. Via Archie Andrews most other Archie characters, including the MLJ superheroes, are thus linked to Marvel.

Marvel to Wildstorm - reality merger - having separated from the larger Image reality, and before merging into the DC main reality, Wildstorm briefly merged with the Heroes Reborn version of Marvel's heroes.

Marvel to Warrenverse - reality jump - Warren's Darklon the Mystic, now noticably older, resides in 616 as Darklore. Semi-authorized as Darklore's creator, Jim Starlin, also created Darklon.

Marvel to Ultraverse - reality jumps - Thor traveled to the Ultraverse in Godwheel#3, the first (?) of many crossovers between the two.

Marvel to Image - reality jumps - there were definitely cameos before tying the two together, with Marvel characters turning up for a wedding in Savage Dragon, as prisoners in Spawn, etc. or someone who looks like Officer Dragon turning up in Spider-Man Team-Up, but the strongest links between the two realities would probably be Invincible meeting Spider-Man in Marvel Team-Up II#14.

Marvel to Gold Key - Laralei, a character from Dagar the Invincible, turns up in Kull the Destroyer. Since Dagar has crossed over with other Gold Key characters such as Doctor Spektor, this ties in the majority of Gold Key characters.

Marvel to Dark Shadows - temporary merger? In Dark Shadows I#34, the evil wizard Morath, foe of Barnabus Collins, encounters Agamotto and visits Dr. Strange's Sanctum.

Marvel to Raffles - incorporated into Marvel, as Raffles teamed up with the android Human Torch in All-Winners Comics#8

Marvel to Fu Manchu - incorporated into Marvel in Special Marvel Edition#15.

Marvel to KISS - reality jump as the band came to 616 in Howard the Duck I#12.

Marvel to Judge Dredd - blended reality - Dredd shares scenes with Marvel and DC characters in the Comic Relief Comic. Through Dredd you can also get to many other 2000AD series

Marvel to Fawcett - unofficial blended reality - Captain Marvel (Batson, pre-DC ownership) and the android Human Torch shared an adventure in a probably unauthorized Brazilian comic; I say probably, because iirc the title did have the rights to reprint both, but not necessarily to do a new story, especially one where they meet.

Marvel to Raj Comics - unofficial blended reality - India's Raj Comics produced a very unauthorized but otherwise respectful story where their most popular character, Nagraj, met Spider-Man, Superman and Batman. No attempt was made to disguise the characters, so this was an unauthorized appearance rather than pastiches or analogues.

Marvel to Top 10 - reality jumps - lots of analogue characters, but the real links are the Marvel characters seen in the Transworld Transport Terminus in Top 10#8, including Captain Albion, Captain England, Howard the Duck, Nightmare, Loki and some of the Age of Apocalypse X-Men

Marvel to Shadowline - reality jump - while published and owned by Marvel, it's not safe to automatically assume a given imprint line is automatically connected to 616. However, we've got confirmation of that for Shadowline, as Shreck from that reality has been confirmed to be Terror in 616.



More to follow.