I can only pick 5, which is nowhere near enough, so I'm listing my favorites here, instead. Oddly, they don't have every cover online, so I'm just listing the ones they do -- so you can look at them, and not realize how much of a geek I am digging into my collection (what, no Alien Legion? No Deathlok?).
In no particular order:
- Alpha Flight #1 -- John Byrne sentimentality.
- Amazing Fantasy #15 -- First appearance of Spider-Man.
- Astonishing X-Men -- #3 (John Cassaday's leaping Wolverine, back in the yellows) and his #4 (return of Colossus).
- Avengers #503 -- The "Avengers Disassembled" cover, with Captain America sitting on the floor amidst Ant Man's helmet, Mjolnir (Thor's Hammer), Hawkeye's bow and arrows, the Scarlet Witch's headband, Iron Man's helmet, and the visions cape. Sad and weighty.
- Captain American -- #248, #250, #254 are John Byrne covers and Roger Stern arcs. Ish #248 was actually one of my first comics as a kid (and then I went back-catalog as an adult), #250 is the "Cap for President" cover that graces one of my favorite political Ts, and #254 is Baron Blood and Union Jack. Oh, and Mike Zeck's Annual #8, with Wolverine in his brown duds sparking off of Cap's shield.
- Captain America (2004 series) -- There are sooo many good Steve Epting covers here. I'm particular fond of the gravitas of Captain America at the graveyard in #4, the powerful S.H.I.E.L.D. cover of #9, both the bloody jersey and newspaper Epting variants of #25, and the Alex Ross variant of the not-yet-release issue #600.
- Daredevil -- #183, with Punisher unloading into Daredevil, shortly before Punisher crossovers became obnoxious. And there's David Mazzucchelli's excellent #228, which vividly shows Frank Miller's deconstruction of Matt Murdock, and his cover for #233, which is a powerful issue where Daredevil gets done with the Kingpin.
- Devil Dinosaur #1 -- Another of my first comics as a kid, and Jack Kirby owned me.
- Excalibur (first series) -- Wow, Alan Davis, Ron Lim, and Rick Leonard did some great, fun covers. I really dig Davis's #1 (team intro) and his wraparound #4 (first of the cross-time caper arc). Oh, and #23's riff on "Days of Future Past".
- Giant-Size X-Men -- First of the new team, and the long-time core members many of us remember as "the" X-Men.
- Havok & Wolverine: Meltdown #1 -- I can't remember who painted this cover, but it is gorgeous.
- Howard the Duck #1 -- I think this is a Frank Brunner cover, and it's got Howard, Red Sonja, and Spider-Man. Yes it does.
- Immortal Iron Fist #1 -- Dunno, but for me this David Aja cover just captures who Iron Fist is.
- The Incredibly Hulk #340, #345 -- I dig both McFarlane's Wolverine cover, and his gray Hulk smashing through the logotype.
- Iron Man -- For me, John Romita's cover of issue #126 is the Iron Man armor, and Bob Layton's gutsy alcoholic Tony Stark cover for issue #128 really sticks with me. I like Mark Bright's cover of #225, because even though I don't like the silver centurion armor, I'm oddly sentimental about those issues. Maybe because that was the height of my collecting before it went near-dormant for a few years (girls were prettier).
- Mighty Avengers #2 -- It's a Frank Cho female Ultron.
- New Avengers -- They're missing virtually all covers on Marvel.com, including some Frank Cho covers. Egregious.
- New Mutants -- All of the Bill Sienkiewicz covers (18-25). I would add his Daredevil graphic novel cover, but it's not listed.
- Secret Invasion #8 -- Can't remember the artist, but it's a great nod to New Avengers to Avengers proper.
- Secret Wars #5 -- Spidey's black costume!
- Spider-Man: Blue #2 and #3 -- Man, I love Tim Sale's 60s spy movie poster sexy vibe.
- Spider-Woman #1 -- Another of my first comics, with a cover by Joe Sinnott, so sentimental as hell. I just realized this is the only comic from my initial collection I haven't re-purchased.
- Stormbreaker: The Saga of Beta Ray Bill -- I'm a ridiculous Beta Ray Bill fan, and while this series ultimately let me down, the covers of #1 and #2 -- by Andrea Vito and showcasing the raw majesty of this character -- certainly did not.
- Thor (first series) -- Sooo many good covers, but none (none!) like Walt Simonson's #337 (intro of Beta Ray Bill) and #366 (Frog Thor). (I like the Olivier Coipel covers of the new series ok, too.)
- Uncanny X-Men -- Long ride here, but I remember Dave Cockrum's #100 (Charles Xavier standing), #101 (Phoenix), #109 (intro of Alpha Flight), and #112 (why Wolverine should never fight Magneto). And John Byrne's "The Day the X-Men Died" cover and death of Jean Grey (#114 and #136) are alternately very sad, simple and powerful, angsty covers, as opposed to his very dynamically powerful "Days of Future Past" covers in #141 and #142. Paul Smith's cover for #167 (death of Professor X) is a solid nod back to #136. Alan Davis's cover for #213 is what I always think of as Wolverine versus Sabertooth. I so dig Jim Lee's cover to #268, which is a powerful portrait of Captain America, Wolverine, and Black Widow.
- Web of Spider-Man -- The Mike Zeck covers for issues #31 and #32 of the "Kraven's Last Hunt" arc (some of my favorite writing from J. Dematties) really capture the essence of that twisted story.
- Wolverine #1 -- Frank Miller's beckoning cover paints a portrait of the deadly, off-kilter canuck.
- X-Men #1 -- I so like how this Jim Lee cover brought Magneto back as the terrifying force of nature he should be.
- X-Men: Magneto Testament #5 -- This Marko Dhurdjevic cover is heartbreaking, and is for me the best in the series.
That's the list for now. Too many covers aren't listed on Marvel for the vote, so I'm bummed to not be talking about some of my other favs.
But this is a good nod to people who have inspired be over the years.