Anything that is titled Hellsing Ultimate is correct
OVA = Original Version Anime
Basically referring to the fact that it follows the manga (in other words the anime follows the original story). The reason OVA is added to some anime titles, is because there might already be an anime out there, but doesn't follow the original story.
So, with Hellsing for example: You have already an anime series named Hellsing. Just Hellsing, no add-ons or anything. However, I would highly discourage watching that. I watched and rewatched it. I didn't know at the time about the manga, and I surely didn't know it didn't follow it either. Having rewatched it a few years after my initial first time watching it, I thought it to be rather tasteless. No joke. The music is cool and whatnot, but the story... The first three episodes, if I recall correctly, of the Hellsing series does follow the manga, but after that it just takes a different turn, and goes completely downhill from there.
AT least is what I recall. And the (main) villain is definitely not the same either.
Hellsing Ultimate is, in my opinion, very well done. A lot of time went into it. And I mean A LOT, check the dates of when each episode was released, months, sometimes a year or so was in between them. That had a lot of reasons. But they wanted (and did) use computer animated graphics, but only at certain points. Implementing rather well in the whole animated/cartoon section.
When you watch it, I would highly suggest to pay attention to when they use CGI, I think it's pretty neat, personally. All the details put into it, makes it an amazing show to watch.
Sure, storywise I think it's pretty cool too. For an anime bieng in my toplist, it has to not only be visually pretty, it mostly need to have a pretty nice storyline. Doesn't have to be complicated. As long as characters get background. That their personalities are defined.
And that is what happpens in the series of Hellsing Ultimate. In the first episode you already get a backstory to one of the main characters (Sir Integra Fairbrooks Winsgate Hellsing). They don't just show a bit and never further show anything, they keep showing flashback and other various things to define a variety of characters. Some have more backstory, others less, but when a character acts in a certain way, it makes sense. There is always a reason for it.
That is what I like
There, I think I haven't given any spoilers whatsoever
As for dubbed and subbed:
When I (or anyone else) talks about watching an anime subbed, it is referred to watching the original version (usually Japanese audio) with English subtitles.
When mentioning dubbed, it is usually referred to watching the English translated version (so no subtitles).
That is why I would suggest, USUALLY, to watch an anime subbed. I always watched anime dubbed before I found out the subbed versions usually sound nicer. As usually the dubbed versions just have American voice actors, and all have similar tones with their emotions, etc. That's my personal experience.
However, with Hellsing Ultimate I suggest watching the dubbed version.
Why?
Because the character who are shown to be British have been given a British accent. My favourite character Maxwell is Italian. Guess what accent they give him?
Right, an Italian accent.
I'm not saying that the accent of all of them are amazing or great, however, it is EXTREMELY enjoyable for me to watch. It feels they really tried to put in effort, and the accents are well done overall.
Hope I answered all of your questions