[merylsilverburg]

Well, Akira is one of those masterpieces which everyone should own or at least see. Own it, if you definitely love it. Laputa, I haven't seen yet (the subtitles were irritating me for some reason) but the most reaction from a lot of people is that it's very much more like a children's film as opposed to Mononoke-hime or "Spirited Away". However, if you don't mind that sort of thing, then Laputa would be a good buy, since it's considered one of Miyazaki's greatest. Of course, like Sodge said, getting both wouldn't be bad if you can afford it since both films are great.

kupo, is "Nausicaa: Valley of the Wind" (Kaze no Tani no Nausicaa) released over in Europe early? If so, I'd recommend you check it out or get this film instead. Since you love Mononoke so much, Nausicaa should appeal to you since this film is an earlier version of Mononoke. It's also much, much better IMO, despite it being outdated in the visual area.
Also, check out "Porco Rosso" (Kurenai no Buta) if you can...I really love this film...it's more mature/older orientated but done really well...it's a great film, 'nuff said. I also heard that "The Cat Returns" (Neko no Ongaeshi) has been shown in France recently...hopefully it'll make it to the UK...this film, though not an "official" Ghibli film (produced by them, but not directed by Miyazaki or Takahata) this is one of my favorites. It's an adventure like "Spirited Away" but more shorter and involving cats. Don't ask me why I love this film so...I just do. I'm not suggesting you put this on your "definite" check-out list, since it might not appeal to many, but it's worth a look if you have time to spare.

Other films which I think are worth your time: (if you enjoy all genres instead of just action or mecha, etc.)

Vampire Hunter D and Vampire Hunter D: Bloodlust: Though I'd say get the original, classic version for the great story, the modern one might be more appealing if you like more updated visuals...with an okay story...of course, it's all in opinions, so you'll just have to see which one appeals to you more.

"Millenium Actress" (Sennen Joyu): Directed by Kon, Satoshi and created by the team who did "Perfect Blue" (also by Kon, which you should also see if you like thriller, action-drama films which a more realistic visual style), this one is a quiet, "romantic" film that deals with the story of an actress' life imitating art. Completely opposite of "Blue", this one really is outstanding IMO. The recent one to hit States (and I think Europe as well?) is "Tokyo Godfathers" and I do like this one but it doesn't "shine" out to me as much as "Actress". Visually better than the other two films, retaining that same "story-telling" style of Kon's films.

There's also the very popular suggestion: Ninja Scroll if you haven't seen it already. It's right up there with Akira (maybe a notch or two down, depending on who you're talking to) and it's just an all around, great film which mature tones, scenes, gore, etc. It's also a classic which everyone has seen at least once. The sequel Ninja Resurrection is one to be avoided at all costs.

Speaking of classics, there's also the "Ghost in the Shell" movie (the first one) which is completely outdated once you see "Innocence" but it's the original film. Since you didn't like Metropolis (regarding androids and humans), I take it you might not like this one as much, but the visual style is nice but as said before, older. Of course, I can't say much more since I never saw "Ghost" or "Metropolis", so it's better to ask the others on here.

Hmm...there's also "Blood: The Last Vampire" but this film is one of those "like it, hate it" types. Apparently left off with a cliffhanger that drove a lot of people nuts (wanting to know more), the visuals are supposed to be very top-notch and beautiful. The story is probably a bit unbalanced since it was based off the game, but I think if you enjoy just "mindless action", it shouldn't be a huge problem.

Ahh, there's more but I can't seem to think of them right now. Hope this helps with your future purchases, at least.