Your poll is missing the 80s...tons of great OVAs from the 80s. An aside, when I first got into anime fandom "OAV" seemed to be the more common term. Perhaps it was an English fandom thing? When more people started calling them OVAs, the excuse I heard was that OAV implied it was a porno.
Anyway, my all time favorite OVA is
Gunbuster. That is the show that made me an anime fan. I had watched a few of the early North American commercial VHS releases (this was in the very early 90s before I had ever seen a fansub, so my exposure to anime was low), and none of them really caught my fancy beyond being something different from Western animated material. Then came the US Renditions VHS release of Gunbuster (around 1991 or so?). I first saw this when the Mom 'n Pop rental shop (remember those?) I used to frequent in my early college days got the first tape in. It blew me away and has remained one of my top anime to this day. This was one of the first pieces of anime that I purchased ($40 a tape for two episodes, yeah).
As for some underrated OVAs, I always thought
Kishin Corps was one. It always seemed to be overshadowed by
Giant Robo. I am not a mecha fan at all; if I watch mecha anime the mecha are secondary to other elements (plot, characters, etc.) However, I love the mecha in Kishin Corps. Watching this series I can understand those fans that obsess over the design of mecha in other shows.
Speaking of dubs, back then I avoided dubs like the plague, because many were quite bad. Plus, with VHS you had to choose, and getting stuck with a crappy dub hurt when you were already paying at least $20-30 for a one or two episode tape. After getting burned on really bad early dubs (like Bubblegum Crisis) I swore them all off. However, when US companies started releasing bilingual laserdiscs (and I got into LD in a big way in the mid-90s), I started listening to a few dubs and actually liked some of them. Dubbed OVAs I liked included:
Oh My Goddess - the OVA from 93-94 is the only part of this franchise I like. And the dub was quite good, I am willing to listen to either the Japanese or English on this one depending on my mood.
You're Under Arrest - again, the OVA is the only part of YUA that I like. I actually prefer the English dub for this.
Macross Plus - I still can't believe the dad from Malcom in the Middle voices Isamu.
El-Hazard: The Magnificent World - another one where I prefer the English dub entirely (I think I've only watched the Japanese once).
And here's a few other OVAs that I liked back in the day that weren't on your list:
Gundam 0080: War in the Pocket - the only Gundam that I really enjoyed (and again, it wasn't because of the mecha).
Patlabor - I'm listing a lot of mecha shows, but I like them for other reasons I swear!
Video Girl Ai - seriously truncated from the manga, but I still loved it. Also liked DNA2.
Here is Greenwood - I always wondered why this OVA received two different English dubs. I think I have a soft spot for this because Minaguchi Yuko voices a character in episode 4, and I always loved her voice (Inokuma Yawara, what a sweetheart).
Ambassador Magma - I liked this because when I was a kid I had really enjoyed the live action "Space Giants" dubbed version of Magma Taishi. However, when I got older no one I knew had ever heard of "The Space Giants" and wondered if it was some figment of my childhood imagination until I saw this OVA.
Key the Metal Idol - the first "trippy" anime I got into.
New Cutie Honey - has one of my favorite ED songs "Circle Game" for the first two episodes.
Please Save My Earth - another OVA that really chopped up the manga, but still great.
I've said before but it's really a shame the OVA has mostly gone away and been replaced by one cour TV. A lot of one cour TV shows would be better off if they were shortened down to six episodes and had higher production values and less filler (yeah, filler on a 12-13 episode show is pretty sad).