Austen's X-men
Seconded on this! US 1 has limitless potential.
As for the worst:
Ultimatum (Loeb's utter destruction of the Ultimate Universe... I still cannot forgive this)
Civil War II (the most needless, pointless "event" in recent memory that couldn't even properly leverage the concept)
America (nuked an otherwise promising character)
Patsy Walker, A.K.A. Hellcat (just awful)
Sins Past (for obvious reasons)
IvX (trying to make Inhumans happen...)
Age of Marvels and DC Next Dawn - Monthly Fan Made Solicitation Competitions on these very forums, make your pulls now! Want back story? Check the Wiki!
Spider-man: Chapter One is a retelling of the first few issues of Spider-man, except with far worse character designs, pacing, and dialogue. It makes a bunch of stupid retcons and lacks ANY of the charm that Stan and Steve brought to the stories.
Spider-man: Get Kraven. This whole series is just a call out to the writer's Hollywood friends. It's almost-sort-of a comedy, except it's not funny. It's a rambling story told by somebody that thinks they're a lot funnier than they really are.
Wolverine's no-nose phase
Warren Elli's 90s Thor run 9and the surrounding issues) Thor is suddenly mortal for some reason and decided to walk around with his shirt off all the time and sleep with Enchantress. He also drops all his "Thor-speak" and talks like he's lived in New York all his life. There's almost a plot that involves the World Tree.
I'm also going to pick on Warren Ellis again for his X-Force run, where he obviously didn't know anything about the characters, shoe horned in his self-insert Mary Sue (Pete Wisdom) as the new leader that X-Force is just drooling all over, replaced all the characters outfits with black leather and gave us an artists that made Liefeld's work look great.
Also Civil War and Civil War II. Civil War was a horrible thought out event where almost everybody was written out of character, no two writers seemed to be in agreement with what was going on, and it was responsible for making almost the entire Marvel Universe unreadable for a while. Civil War II ... actually I don't know what happened in Civl War II. Nobody does. if you ask around, people will mention something about characters acting like they've never met a time traveler before and deciding to reinact Minority report, but the story itself was so unmemorable that you're not going to get much of any other explanation. Oh, wait. One more thing. It contained the classic Bendis "I'm out of story now, so let's have a dues ex machina come in to wrap things up."
* Everything done by Chuck Austen, even more when you take in consideration the awesome runs that were before him (Morrison in X-men, Geoff Johns in Avengers).
* Ultimatum by Jeph Loeb and David Finch
* The clone saga, 90s version
* Sins past
Bonus: The ultraverse, Marvel version.
The Amazing Spider-Man: One More Day.
The Amazing Spider-Man: Sins Past.
Civil War II.
The Thanos series from 2003 is so bad. Starlin's one 'flop' in my opinion. It's a slog.
Others have picked some of my choices for THE worse so here are a few others:
There was a Speedball story in the 1990s Marvel Super-Heroes series where two men murder each other over a toy gun called the Evilizer.
New Avengers by Hickman had heroes standing around debating the morality of destroying other Earths to saved their own. I actually felt myself becoming depressed reading it.
Marvel Tarot was a rare miss among Marvel reference works. If you didn't already have a passion for the tarot, it required more concentration than it was worth to make out what was being said.
Adminstrator,
Who Watches the Watchers - Forum for the Official Handbook of the Marvel Universe & Similar Works
https://whowatchesthewatchers.boardhost.com/index.php
While the "New Avengers" series it led into was good, "Avengers Disassembled" is like four issues of everything that was wrong with superhero comics in 2004. The ugly, teeth-clenched art, the constant explosions and washed-out "realistic" colors, the "mystery" that doesn't even try to be a good mystery, and of course, characters being killed off or turned into murderers for shock value. The corniest superhero story from the Golden Age is less dated.
Ultimate Power. It's a story padded by Bendis and drawn by Land. Nuff said.
Avengers #200. If this comic was a person, I would kill it.
Marville. It's Marville. I don't need to say anymore.