Samurai Champloo
Excellent high-quality animation from beginning to end. Pretty dramatic, with some down-to-earth humor mixed in.
It's got the quality of Sword of The Stranger in terms of production value, got a story to roll with big media (I.E not just "good for an anime"-kind of story), and it's got a soundtrack produced in part by Nujabes, one of the best jazz/chill-out/hip-hop producers to come out of Japan ever.
Ghost in The Shell - Stand Alone Complex
I know you wanted reasons, but it's Ghost in The Shell. I mean, like seriously. Not having seen GiTS - S.A.C being a fan of anime, is like not having watched Blade-Runner and being a fan of Sci-fi.
Noein - To Your Other Self (Noein mou hitori no kimi e)
Great animation quality, quirky story with quantum mechanics, time-traveling, lots of action, and a lit bit of romance.
Seeing as you liked Ergo Proxy, I think you'll like this, even if it's completely different. There's just something about the mood of the series.
Well, it was directed by Akane Kazuki, who has worked on loads of the anime you listed as having liked - lots of Gundam, Ergo Proxy and The Visions of Escaflowne (he also worked on Samurai Champloo btw).
Tower of Druaga
Again another really high-quality anime in terms of production value
(I don't watch anime that looks low-budget in general though, and in terms of art style, you'll notice that most of these are consistent as well. No loli/chibi/or weirdly disproportionate characters).
Two seasons of swashbuckler adventures of a motley crew of heroes climbing a giant tower.
It sets the stage with a really simple and banal premise, and then compliments it all with tons of character and quirks.
Looks silly on the surface, but has a lot of depth underneath, so in a sense quite similar to Tengen Toppa Gurren Lagann.
Wolf's Rain
One of the heaviest and heartwarming anime I can think of. It's in many ways more akin to a Disney journey than your average anime though (that being said it's much darker than most Disney productions for sure). It's a series about a bunch of wolfs/kids(like wolfs who can turn into humans) who leave a dystopiac future city in search for "paradise".
Again, high production values with a soundtrack by Yoko Kanno (same producer as GiTS - S.A.C), one of the best and most famous composers and producers in Japan.
It just occurred to me that I pretty much know all the high-end Seinen/Shounen-bordering-on-Seinen anime titles, and that most of these are in fact pretty good, so I'm just going to list the last ones after this point with a short comment, because if you've seen (or end up seeing) pretty much any of the ones I've listed so far, you'll know what my standards are, and that I'm not going to put anything in this post I don't feel is consistent with that standard.
Black Lagoon
(action - follows a bunch of pirate mercenaries that smuggle goods in and around the seas of Southeast Asia in the early to mid 1990s)
SPEED GRAPHER
(weird mystery "horror" action about a photographer who gains the power to kill people by taking their pictures)
DOGS/BULLETS&CARNAGE
(title says it all - a excuse for stylish and artsy violence to extreme degrees)
Kemonozume
(Awesome. Has a really strange and trippy art-style, but great in almost every way possible. It's about a Japanese family devoted to hunting down and killing demons in modern day Tokyo, with this very strange Romeo x Juliette plot)
Tokyo Majin
(more demon-fighting in Tokyo. Animation is top-notch)
KARASU
(like Tokyo Majin, except with even higher production values and a main-character that harkens back to the ranger traditions (sentai/power rangers) which sounds corny, but is actually really awesome in this context)
Gun X Sword
(You said you liked Trigun. Well, this is kinda like Trigun all over again, just dressed up in new colors, and giant robots.)
And lastly, two special mentions
Eureka Seven and Scrapped Princess
Both of these are 5-star, high-quality projects with some really emotional stories (the first is a sci-fi adventure with giant robots and airship "pirates", and the latter is a strange fantasy hero's journey that evolves into something completely different as you go along)
I put these two at the end, because they deviate a lot from my earlier choices in appearing much more casual and juvenile when they start up, and they're both relatively long (50 and 24 episodes respectively) with that taken into account.
However, both of these series take a spectacular turn into something much more mature and captures a much larger spectrum of themes and emotions about half-way through, which is why I think both of these are worth watching as well.
That's pretty much it from the top of my head, since you've already seen Gankutsuou (one of my all-time favorites), and since you already have Psycho-Pass on your to-watch list.
(Berserk and Gantz are also two really good stories, but unfortunately neither of the anime do the manga justice, and thus they don't make my list of recommendations).
