Some exemples for me:

Flash Gordon: Dan Barry

The Phantom: Lee Falk and Sy Barry

Spider-Man: Ross Andru, with Gerry Conway or Len Wein, i don't know if i first read a Conway or a Wein story. There were both that Day. + reprint albums of Lee and Ditko.

The FF: Lee and Kirby.

The X-Men: Claremont, Byrne, Austin

Iron Man: Mantlo and Tuska (Midas saga)

Daredevil: Steve Gerber and Bob Brown (the Man-Thing / Gladiator/ Death-Stalker story)

Batman: Len Wein with Don Newton and Irv Novick, also some late Denny O'Neil stories, with the same artists.

Iron Fist: Claremont & Byrne.

Captain Marvel: Doug Moench & Pat Broderick.

Star Wars: Archie Goodwinn, Carmine Infantino and Terry Austin.

The Invaders: Roy Thomas and Frank Robbins.

Captain America: Steve Englehart & Sal Buscema (for his on series, but i first met him in Iron Fist or the Invaders, both series were in the same book.)

The Avengers: Lee & Kirby for their own series, but i had read the Marvel two-in-one issue drawn by Frank Miller a few moth earlier, and i may have seen cameos elsewhere. Maybe in Iron Man.

Thor: Kirby, Lieber & Lee.

The Flash... in the World's Greatest Super-Heroes comicstrip by Martin Pasko and Georges Tuska. When i read his own series, Cary Bates and Irv Novick where on it, but i already knew the character by the newspaper comicstrip. I Just didn't knew his costume was red.

H for Heroes: Marv Wolfman and Carmine Infantino.

As best I can recall...

Captain America: Jack Kirby (during the Captain America & Falcon era from the mid-seventies)

Superman: Almost certainly Cary Bates and Curt Swan in Action Comics

Batman: Possibly Jim Aparo and Bob Haney in The Brave and the Bold

Going by earliest I can remember, and not necessarily the first time I "met" the characters. I had older brothers, and even though I was the only one of my siblings that went on to actually collect comics, comic books were ever present as long as I can recall, making it very difficult to know what was my first comics. So, here goes with what I can recall... (Note, any Silver-Age references are due to reprints I read during my youth.)

Spider-Man: Lee, Ditko, Romita Sr., Gil Kane, Jim Mooney, Sal Buscema, Ross Andru .. It was a book by one or more of those fellows.

The FF: It was either "FF" #110 or a Lee and Kirby ("Marvel's Greatest Comics" reprints)

The X-Men: (original team) I believe Gil Kane and Gerry Conway ("Marvel Team-Up" #4), then a reprint of X-Men #3 by Lee and Kirby. (New team) Claremont, Byrne, and Austin (#122).

Iron Man: Likely #133 by Bill Mantlo, Herb Trimpe, Keith Pollard.

Daredevil: I remember reading Frank Miller's early issues, but I am pretty sure I saw the character in other stories before that point.

Batman: Bob Haney and Jim Aparo on "The Brave and The Bold," if not a Batman solo comic, also would have been by Aparo.

Iron Fist: Claremont & Byrne.

Captain Marvel: Starlin.

Star Wars: Roy Thomas and Howard Chaykin (read it as the series began).

The Invaders: Roy Thomas and Frank Robbins.

Captain America: Steve Englehart & Sal Buscema, likely.

The Avengers: Pretty sure it was Jim Shooter and JB, the Count Nefaria storyline.

Incredible Hulk: Either Herb Trimpe (the issue with the Glob stands out), or reprints of Lee, Kirby, Ditko in a Fireside publication from the 1970s.

Superman: Curt Swan and likely Cary Bates, but it could have also been  Swan Silver-Age reprint.

Green Lantern: Mike Grell and, I think, Denny O'Neill. It was the period when Hal had Itty the starfish on his shoulder. I thought that was a regular part of GL's character, that critter, like his sidekick. LOL!

Thor: Lee and Kirby reprints.

Legion of Super-Heroes: Mike Grell era, not sure of the writer.

This took a little digging on my part, and the memory banks are a bit iffy after all of this time, but here is what I came up with…

Avengers – this is a toss-up between John Byrne and David Michelinie on issue #190 (Avengers on trial against Peter Gyrich) or Don Newton and Bill Mantlo on annual #9 (Avengers vs Arsenal). I can’t remember which one I read first.

Batman – John Calnan and Gerry Conway, issue #306 of his own series (Batman vs the Black Spider)

Captain America – John Byrne and Roger Stern, issue #248 (Cap vs Dragon Man)

Conan – John Buscema and Roy Thomas, issue #110

Daredevil – Frank Miller’s first issue, #158 (DD vs Deathstalker)

Fantastic Four – Jack Kirby and Stan Lee, via issue #36 of Marvel’s Greatest Comics (reprint of FF #49)

Flash – Alex Saviuk and Cary Bates, issue #278 of his own series

Hulk – Sal Buscema and Roger Stern, issue #239 of his own series

Iron Man – John Romita, Jr. and David Michelinie, issue #128 of his own series (Demon in a Bottle)

Justice League of America – Elliot Maggin (plot) with Dick Dillin and Frank McLaughlin (artwork), issue #119

Power Man / Iron Fist – Kerry Gammill and Mary Jo Duffy, issue #65

Spider-Man – Keith Pollard and Marv Wolfman, issue #200 of Amazing Spider-Man (The Spider and the Burglar)

Superman – Curt Swan and Martin Pasko, issue #332 of his own series

Thor – Keith Pollard and Mark Gruenwald, issue #290 of his own series (about halfway through The Celestial Saga)

Wonder Woman – Jose Delbo and Gerry Conway, issue #259 of her own series

X-Men – Byrne and Claremont, issue #138 (Elegy)

Superman: Curt Swan

Superboy: Kurt Schaffenberger

Batman: Jim Aparo

Spider-Man: Frank Miller 

Hulk: Sal Buscema

She-Hulk: John Buscema

Daredevil: Frank Miller

X-Men: John Byrne

The Phantom: Sy Barry

Teen Titans: George Perez

X-Men was Lee/Kirby

FF was Lee/Kirby

Hulk was Goodwin/Trimpe

Daredevil was Lee/Romita

Iron Man was Friedrich/Tuska

Spider-Man was Lee/Lieber

Captain Marvel was Wein/Kane

I had some old UK DC comics or annuals and in these I think it would be as follows :

Batman was O'Neil/Novick

Superman was Pasko/Swan

At the time I would have had no idea about these creative teams, but I can remember the covers and stories and have pieced it together.

Superman: Dan Jurgens

Silver Surfer: Ron Lim

Adam Warlock: Jim Starlin wrote, Tom Raney was the artist

My dad had a copy of the Origin of Marvel Comics which I think must have been where I first met Thor, FF, Spider-Man, Hulk, etc. 

(I didn't read much DC until later.)