1. Ultima V - PC2. Ultima VI - PC3. Might and Magic VI - PC4. Realms of Arkania: Blade of Destiny - PCOoof, this was a rough ride. Blade of Destiny shows why you can't just translate a tabletop game directly to a computer and expect it to be as good. If it weren't for certain conveniences and the overall low key of the adventure I probably wouldn't have managed to make it. And there's two more games in the trilogy (though I hear the third game fixes a lot of the issues).
Realms of Arkania is a German pen & paper RPG that came about because the D&D licence was too expensive, so they made their own. While D&D (especially that era's D&D) is a pretty abstract system, Realms of Arkania is a pretty involved system. At character creation time you set your seven attributes and then seven negative attributes, followed by using twenty attempts to improve your skills (and if you're a caster there's also tens of attempts to improve spells, and the ability to exchange some of those for either casting resource or regular skills). It's a shockingly involved system, and the character archetypes have very strict and high requirements; it's likely for you to roll up a character that can't be played and have to try again. What makes it especially tough is the fact that the way it works is you roll 1D6, add 7, and that's your stat number, then you pick one of your seven stats to apply it to (negative stats are 1D6 + 1). So on the fly you have to decide to take a good number or hold out for a better one on a key stat. Also, increasing skills allows you only a limited number of increases per skill, and three failures without an increase locks you out. At level up time you go through the same thing, only instead of setting all your attributes you get to increase one positive one and have a chance at decreasing one negative one; all the skill and spell stuff is the same. Since you are likely to have the whole party level up at once this makes for quite the time consuming operation. At least the level progression is D&D style; I was level 4 before the final fight and went to 5 after beating it.
The myriad of skills cover a wide range. In addition to the various weapon skills, you have skills for climbing, swimming, riding, some abstract self control, hunting, finding your way, various lore skills, curing disease and poison, lockpicking, pickpocketing, and many many more. And a large portion of these do have some use in game. You better make sure you train up someone with the appropriate outdoorsman skills or you will starve to death in the wilderness. The game is heavy on the simulation aspect; diseases are common (though in my case it tended to be due to a botched healing roll) and you will spend several days traversing between towns in a cycle of travel, maybe get some kind of encounter (not necessarily a fight), then camp and eat, hunt, gather herbs, etc. I understand that if you do something like walk around without pants or shoes then you're more likely to catch disease. All things that can make for a potentially added aspect to pen and paper but becomes tedium in a computer game.
But the real thing that is a drag is combat. Combat is done on an isometric grid, with everyone only being able to attack in straight lines. This automatically makes magic and ranged attacks piss poor, as party members block line of sight. Complicating your life is the fact that a character is three and a half squares tall, so it can be difficult to figure out exactly where you are in the melee. There is an auto combat button that has the computer do the task of having all your dudes swing swords, but it also puts your mage into harm's way (and makes you realize they probably weren't the effort). And even then it takes way too long, as to hit always is at the 1st level D&D range, it seems. Lots of whiffs, and sometimes when you whiff you either smack yourself in the face or break your weapon.
All that said, the game story itself is fairly low key. There's a horde of orcs coming to invade (and these orcs are more like neanderthals, desipite the opening movie done by a different company) and you need to find the legendary Blade of Destiny to drive them back. Most of the game is spent following a chain of clues; you talk to person A, who gives you a map piece and tells you about persons B and C. They might have a map piece or just references to additional people. Once you have all the map pieces you do a short dungeon to get the sword, then need to do another dungeon to get the orc battle plan that tells you where and when to go to fight them. Then there's a one on one combat between your sword wielder (hope you trained up a warrior with good sword skill) and their champion, and when you win that's game. The two dungeons I mentioned are the only two that are required to win, though you'll likely do two more to get map fragments.
The game was developed with a member of the PnP design team on staff, and he stayed through the second game. As I understand it, he left before the third game and that let them relax things to be more suitable for a computer game. A lot of DM conveniences are waived in this game; towns are rather large that are mostly filled with useless houses, but a third of the towns have important NPCs in those houses. You will learn to hate the face of the "nothing useful in this house" NPC with his giant ears. A tabletop game would shortcut you to "you wandered the city until you found the right place". Same with travel; a DM would probably roll once for a possible encounter and have you roll once to see if your outdoorsman gets enough food; here you can have a five day trek with no encounters and just a lot of menu clicking. And the more abstract combat people would use in PnP would make fights much faster and have more strategy, since you could actually use ranged attacks and magic.
Needless to say, I'm in no hurry to rush to the sequel.