I do recommend playing them all on PC if you can. Maybe 3 on NES but other than that the PC version is a lot better.
The "true" version of 1-5 is the Apple II. That's how old they are lol.
There is also a pre-Ultima game called Akalabeth which is freeware. It's not particularly good. In fact it's extremely basic. But it's interesting as a sort of look at how RPGs first began. This game is actually from before Apple II, believe it or not. The combat and vector maze dungeons are copying other games that already existed, but I believe this is the first game to have a sort of "overworld" that you can walk around in and enter dungeons and other points of interest. And anyway it's free so you might as well download it and give it a try. If nothing else it will give you a look at what the first RPGs were like.
Ultima 1 and 2 are PC only.
Ultima 3 has NES and PC versions. I've beaten both. The NES version looks better and has music, so it's better in that regard. It's also more intuitive if you're used to NES RPGs and not old console ones. But the PC version is smoother, quicker, and more streamlined if you're used to keyboard hot keys, which you should get used to if you want to play Ultima 1-5. So this game will help you get accustomed to the way you play the other games. Overall I'd recommend the NES version but the PC version has its merits.
Ultima 4, the Sega and NES versions have better graphics and sound but the dialogue is dumbed down a lot. And one of the most charming things about this game is its dialogue, where you meet all these people who are talking about the virtues and what they mean. You lose a lot of this in the console versions. Also on PC there are programs you can use to improve the graphics, add music and better sound, and greatly improve the UI so it's a much better experience overall.
Ultima 5, the NES version is basically unplayable. It looks and plays like garbage. That game was just too much for the system to handle. I tried it out once and it was an absolutely miserable experience, avoid at all costs.
Ultima 6, the SNES version seemed okay but at this point PCs had VGA graphics and sound cards so it's no longer much of an upgrade in that respect. The PC version has a much smoother hybrid keyboard / mouse interface and I think the world loses a lot of its complexity.
Ultima 7, the SNES version is playable but it's garbage compared to the PC version. Stay away from that one too.
There also is an Ultima 7 part 2 which is a whole new adventure and game world using the same engine. It's really good too but only on PC I think.
Ultima 8 and 9 are also on PC.
Worlds of Ultima: The Savage Empire is also well worth playing. Uses the Ultima VI engine, and has some fairly nifty parts to it
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