For me, I would have to say it would have to be a tossup between Sengoku Rance and Rance Quest Magnum. I first played SR way back in the day when Fuwanovel had that awful rip of it that only gave you access to the initial playthrough. That game got me into the Rance series.
When RQM came out, I was stationed over in Japan at the time, so I bought the game. I still have the physical case somewhere around my house. I find it interesting that, if the game box didn't have a big (18) sticker on it (and you couldn't read the Japanese), just going off of the pictures alone, you would have no idea it was a porn game. There is no nudity on the box art at all. Rance Quest Magnum I mean. Rance Quest had nudity on the back. Most Japanese adult visual novels had nudity on the back even if they did or didn't have any on the front.
But enough reminiscing. The reason I say it's tough is that I like Rance Quest Magnum's gameplay more than Sengoku Rance's. That doesn't mean I dislike Sengoku Rance's style of gameplay; I really like Daibanchou as well. I played that game enough to get all the scenes, 5 star all the characters, and beat the unbalanced hell difficulty. I played it so much that I have some pretty optimal starting strats for the District and National chapters. District is less optimal and more trying to reduce the number of payment cycles you receive in order to reduce your loan by that much more. The national chapter strategy I found worked for me the best isn't one I've seen on the wiki which is weird because it wouldn't make sense to not perform the action. I'm still sad that Daibanchou (and the Dai series as a whole) never got a sequel. I enjoyed Daiteikoku as well. Not as much as Daibanchou, but still enjoyable nonetheless. If I remember right, Daiteikoku didn't do all that well, so the series got canned. After TADA left, Alicesoft hasn't done much new stuff. They have the Darkest Dungeon game, but they're only working on a DMM game at the moment with no plans for new stuff as far as I know.
Heh. I remember the first time I came back to Sengoku Rance, I managed to find a copy of my old saves. And boy, I absolutely sucked at the game. It was a couple turns before Oda forms the Demon faction. I basically only got as far as Ieyasu and the Hannies. I don't think I was skilled enough to be able to fight against Takeda/Mouri. I only managed to beat Uesugi because you have an instant-win ability against them. After I started playing it again, I had a better understanding of how to play the game. When I first played, I played too passively and didn't understand strategy. I would only fight for a territory after I had 6 strong units that were fully healed. Tokichirou destroyed more seals than I did (which only happens if you take too long to open a new seal).
Let's see, other favorites I have would be...Kamidori Alchemy Meister. I've beaten it, although I never completed all the paths. I saved Yuela (sword girl) for last since she seems like the main main love interest. I ended up never completing her path, so I don't know how her storyline goes.
Monster Girl Quest is another one of my favorites that I've beaten. As much as I like MGQ, I have not touched Paradox yet. I'm waiting until part 3 comes out and gets translated. As far as I know, all 3 parts can be combined into one like the base game could. So, once it releases, I'll get around to playing that game. I heard it has a shit ton of content. Although, I might play MGQ again just to have a refresher on the storyline.
On the western front, hmm...this is a bit of a trickier situation. Due to how western adult game developers operate, most of them just start a Patreon and string along the development of one game in order to retain their cash flow. Hey, I understand the need to get that bread, but it's resulted in a lot of games that haven't been completed yet. Trials in Tainted Space is a game that would be in contention as one of my favorite western adult games, but that game has been in development for an enormously long time. The very first Patreon post for TiTS was back on March 31, 2014. Even if that were somehow not the first version, it's now been over 10 years that the game has been in development. This is one of my main gripes with how the west does things. As a result, while I do have quite a few favorite western adult games, a lot of them are not completed and so I don't consider myself having "beaten" the game.
There are some western adult games that have been completed and even fewer that I would say are in the running as one of my favorites. Okay, this post has been going for awhile now, so I'll just lightning round them: Once in a Lifetime, Four Elements Trainer, After the Inferno. That's...probably it. There may be more, but I can't think of any more off the top of my head. Again, I'm not saying that I don't have more favorite western adult games, it's just that very few of them are completed (and thus not beaten). The Last Sovereign is also definitely one of my favorites, but again...haven't beaten it yet.