To put my suggestions in perspective, I consider a dub good/great if it both sounds great, as well as has a translation that treats the material well. Generally I don't think 100% accurate translations do well when a show is dubbed, and I appreciate a good translation that is willing to switch out words/idioms/etc to make the dialog feel more natural when spoken in English. It also helps if there weren't any truely iconic Japanese voice actors. For example, I so love Miyuki Sawashiro's voice, that it almost doesn't matter how good an English dub would be for any show she's in.
I should also mention that I couldn't stand most dubs when I first started getting into anime, but after having watched hundreds of shows, I find many (if not most), dubs fine, with some superior to their Japanese counterparts. I suspect this has to do with me becoming used to certain conventions used within the medium.
On to a few suggestions:
Ghost in the Shell Stand Alone Complex (both seasons). One of the best sci-fi/cyberpunk series out there, with one of the best dubs. Pretty heavy-hitting cast of who's who of the English voice acting community.
Haruhi series (season 1, ova) have great dubs. Season 2's dub dropped in quality, though it would likely be considered a good dub if you didn't have the earlier work to compare to. It's not just a reduction in quality of the voice work, but the translation team also dropped the ball in season two, with a less smooth/more awkward dialog (it's a little jarring marathoning both seasons, as the difference in dubs is that noticeable). While Wendee Lee is a great VA, she's been in the business a long time, and her voice has noticeably changed and lost the range it had when she voiced season one. On a lesser note, the same was true for the voice actress who voiced Asahina. The other VAs largely sound the same, and the drop in quality is mostly tied to the translation work on the script. The humor is hit or miss, as the strongly polarized views within the anime community would attest to. I should stress that while I am criticizing season 2's dub, it's relative to the very strong performance of season one (and the humerous omake shorts ova). Season 2's dub is certainly not bad.
Outlaw Star. Always residing in Cowboy Bebop's shadow, Outlaw Star is still a prime example of the good 'ol days of sci-fi adventure in anime. And, imo, its dub is better than Cowboy Bebop's, and all-around great in its own right.
Banner/Crest of the Stars series. Old-school space opera classic. Some people really hated the dub, but I thought they nailed what the Japanese cast was shooting for.
I recently re-watched
Darker than Black in dub format, and I have to say, it was pretty good. Another excellent science-fiction series. Season one is fairly rote (but excellent), with mini arcs running a couple episodes each. Season two is one long arc, with a more human story. This change caused all sorts of angst among the fandom, but I actually really enjoyed season two, because of that change.
R.O.D. the TV has an excellent dub, and is another posterchild of the golden oldies days, with a solid plot/adventure, characters, and voice acting.
Licensed by Royalty has a British cast, and maybe I just have a thing for that accent, but it was one of the first shows I watched where I enjoyed the dub over the original Japanese. The story revolves around two special agents that are obviously based on the UK's MI5/6, called L, and R, with episodic stories that tie into an overarching plot befitting what you'd expect from a show concerning special agents/spies.
I could go on and on, so I'm going to end with:
Baccano! Terrific dub, and terrific show. Takahiro Omori has tried to recapture that magic in shows since, but he's failed every time. This remains his seminal work.
Lastly, here are a few english voice actors/actresses who generally do good voice work:
Wendee Lee - Been doing anime forever. Voice is losing its vigor, but possibly the most prolific female voice actress with the most "good" shows under her belt.
Crispin Freeman - Male version of Wendee Lee
Laura Bailey - I think of her as a (slightly) newer generation/version of Wendee
Mary Elizabeth McGlynn - Not only a unique voice (I think of her uniqueness among english voice actors in a similar vein as I do towards Miyuki Sawashiro for Japanese voice actors), among english voice actors, she's done some outstanding work on directing the english script for a lot of anime greats.
William Frederick Knight - Seminal old-man english voice in anime?
Richard Epcar - Another from the golden days of Lee and Freeman (ok, Freeman is still golden I'd guess). His "Batou" from GITS is probably the most "spot-on" performance in terms of perfectly mimicing his Japanese counterpart, ever.
Cherami Leigh - New generation voice actress. Her only real stumbling block from being as "good" as the olden goldies is the rather paltry amount of great shows she's done. Which is a shame, as she's one of the best young voice actresses.
As voice acting is usually not a career, but a step-stone to another career, some of my favorite english voice acting performances were done by people who only did one or a handful of shows. I'll refrain from listing their names.
