So for me, as a kid, this was the golden age of annuals, where they told stories that no single issue of a regular comic could contain. And the best of them was a 2-parter between 2 different titles' annuals.

So my 5 choices-these were all annual I owned/had access to as a kid so a lot of this is nostalgia-fueled

-Avengers Annual 7/Marvel Two in One Annual 2-I owned one, my cousin the other (until I inherited his comics later when his parents sent him to what passed for rehab then, but I read his annual every time we visited their house and borrowed it between visits when he let me). These may have been the most read comics I had as a kid. I adored this story, it's scope and scale, the many small and big moments it had. It's what made me a fan of Starlin (I know the forums are pretty split on Starlin, but I was always a fan of his)

-Invaders Annual 1-from the glorious Schomburg cover, through the cross-over story within, I loved every page of his. Avengers and Invaders were my two favorite super-teams as a kid, so this was pure heaven for me.

-Thor Annual 6-Thor, the Guardians of the Galaxy (another fave of mine as a kid), and the introduction of Korvac all fueled my imagination as a kid

-Hulk Annual 5-I liked the hulk far more as a kid than I do as an adult, but this was one of my favorite Hulk stories I owned as a kid (The Rampaging Hulk issue I had as a kid was the only one I liked more), it was one of my early introductions to Doctor Strange-a character that intrigued me but confused me as a kid, and late became one of my favorite Marvel characters, and the introduction of Her-I did so much catching up on Marvel lore it was always fun to be in on the ground level of something as a kid

-M

I picked Marvel Two in One #2. But that one and Avengers #7 were parts one and two of the Warlock/Thaos finale.

If I can only go with 5, I have to leave out the Eternals Annual. Anything with Karkas and Reject was golden. However, here are my 5:

Avengers #7 and MTIO #2- climax of the Thanos Saga, with Warlock facing off against the death-lover. Pure classic Starlin, which he mostly revised for the Infinity Gauntlet. As it was, the Soul Gem thing was kind of a revision of the earlier Cosmic Cube Saga. Starlin just kind of kept rewriting the thing, though he had more twists and turns with Warlock added to the mix.

Daredevil Annual #4 is a sentimental favorite, as it featured DD and Black Panther teamed up and battling Sub-Mariner and it was fun stuff. My cousin had a copy and I read it several times, before he gave me all of his comics.

Thor Annual #6, because it was the climax of the Guardians of the Galaxy, as they all battle Korvac.

Avengers #6 because it is George Perez, and has The Whizzer trying to stop the rampage of his son, Nuklo. Plus, The Living Laser!

My cousin had Hulk Annual #5 and I found it to be pretty dull and repetitive.

Thor Annual #5 Essentially a book-length "Tales of Asgard," this issue features a great script by Steve Englehart and gorgeous John Buscema-Tony DeZuniga art.

Iron Man Annual #3 A fun Steve Gerber-penned romp guest-starring Man-Thing. Compare the art here (Sal Buscema inked by Jack Abel) to the gawdawful results when Sal is teamed with Mike Esposito on Team-Up Annual #1 if you need proof that the inker can make a huuuuge difference in the quality of the finished art.

Invaders Annual #1 shows us the events of Avengers #71 through the eyes of the Golden Age heroes. Worth the price of admission just for Alex Schomburg's first comic book art since the late '40s.

Avengers Annual #7/Marvel Two-in-One Annual #2 I don't like Jim Starlin's work, I loathe Thanos, cosmic-themed comics annoy the crap out of me, and I still can't deny the quality or significance of this two-parter. If only Starlin had left this as Thanos' last stand. Sigh.

Cei-U!
By the way, I also own copies of all these annuals (not to brag)!

This is an era I love near my earliest reading days, lots of nostalgia here.

I went with the following:

Marvel Team-Up Annual #1 - Spidey and the X-Men team I loved best, simple as that.
Marvel Team-Up Annual #2 - Spidey and the Hulk, and I always liked Darkstar along with her companions as the villains.
Iron Man Annual #4 - It was always the cover that got me on this one, MODOK plus the Champions (it doesn't get more 70's than that) accompanying ol' Shellhead himself.
FF Annual #11 - Classic team plus the Invaders, what's not to love?
Amazing Spider-Man Annual #10 - I always thought the Fly looked so cool, great looking cover too.

This era was before my time as a collector but I have read several of these over the years. When I was a kid I had Avengers #7 (given to me as a gift, but I was totally lost trying to read it because i had zero prior experience with Starlin cosmic stuff at the time) and FF #11 (which I likely grabbed because I was tricked by the Kirby cover,  assuming his art was also inside the issue). In the last few years I've read Invaders, Eternals, Defenders, and skimmed the Hulk's on Marvel Unlimited (oh, and I also read Avengers #7 and Marvel Team-Up #2 when I went through all of Starlin's cosmic stuff, finally being able to appreciate Avengers #7 after all these years). However, my pick for favorite of the 1975-1977 annuals is curiously missing from the options provided: my favorite 70s Kirby Cap story, the wild science fiction/horror mashup from Captain America Annual #3

Almost all the annuals I chose (except Marvel Two in One which came out in early fall) I got in the late summer during the middle of a heat wave when I had the chicken pox. My dad went to the pharmacy to get Calamine lotion for me and came back with a fat stack of comics for me to read while I endured (I remember Eternals #15 being among them as well and having no idea what was happening in that book, but my dad liked Hulk so he picked it up). Among them were the annuals (my first experience of those) and a handful of other issues. This was the most comics I had ever gotten at one time, I usually got one a week or once every 2 weeks with my allowance, which just covered the price of a single comic, and I hadn't gotten my first polybagged 3 pack yet, so this was a virtual feast of comics for me as a kid. The only other annual I got as a kid was Avengers Annual 9 a few years later, but other than that I didn't get another annual until I was working and making my own money to buy them. The Marvel 2 in One is the one my cousin had that took me some time to get on my own.

-M