I'm not a real critic. I have thoughts, but I'm never gonna write some deep analysis of a thing.
But I know what I like, and Warren Ellis is one of my faves.
He may not have made Watchmen or Klaus, but these are a few of my favourite things Warren Ellis worked on:

Transmetropolitan. Planetary. Nextwave. Injection.
I think it's safe to say these are his top tier works. Especially if Injection gets finished.

Anna Mercury and Doktor Sleepless and Desolation Jones and Fell.
It's truly sad how many of these series just ended mid-story for whatever reason, but what exists of them is still great.

Black Gas and Black Summer and Wolfskin.
Ellis gets challenged to come up with new takes on zombies and superheroes and barbarians and delivers some outstanding stuff.
Black Summer especially feels like a response to Millar, done with more panache.

FreakAngels.
Ellis and Paul Duffield doing a webcomic and this is one of my top tier Ellis comics. I'm mad I don't have the hardcovers and didn't know about it til it was over. Shocked it's taken so long to adapt to other media too.

Ignition City and Ministry of Space and Orbiter and Ocean.
I am a sucker for space shuttle/space exploration stuff, which might be part of why Ellis is one of my faves.
The Orbiter/Ocean Deluxe hardcover DC put out a few years back is *chef's kiss*

Red.
This series was only 3 damn issues and they've made 2 movies based on it starring Bruce Willis.
I'm not saying that's a sign of quality, but it speaks to the strength of the idea.

Crécy.
If you haven't read this one, find a copy. It's a 48-page graphic novel about a battle between France and England and why the English won and it's so well-written and fun to read.

Scars.
This one hits hard. Ellis was challenged to write horror. So he wrote about child abduction and murder, because he had his daughter and that was the scariest thing he could imagine. And I get it.

Shipwreck. More recent, but you can't go wrong with Ellis and Hester.

Global Frequency and Secret Avengers.
There are a lot of similarities here. SA hits harder if you've read GF. Both are amazing, and with so many great artists.

I'll add more Marvel and Wildstorm stuff to this thread later. Gotta go make dinner and spend time outside.

Not gonna list all of Ellis's Marvel greats.
His Iron Man and Moon Knight runs are evergreen.
Thunderbolts is my favourite comic ever, but one of the last things I connect Warren to, which is how much I love his other work.
And he wrote some of the best Ultimate stuff.

And of course there's Wildstorm.
DV8 with Humberto Ramos.
Stormwatch.
The Authority with Hitch.
And The Wild Storm with Jon Davis-Hunt, one of the best comics of the last [unit-of-time].

Writing this thread, I realize there's Ellis stuff I haven't read (Excalibur, Doom 2099, Hellblazer) and other stuff I have but totally forget. Gonna have to rework my reading list to fit some of it in.