Zhyareth

The question in the title and the one in the post are actually two different things.
Regarding the question in the title about the best looking title, this is actually a highly subjective thing. For example, Fata Morgana is regarded by some as a very good looking title because of its unique art style, but its production values are actually quite low. The backgrounds are just blured photographs, its not even voiced and only the main characters have sprites with very little variations.
So what title looks best to YOU is hard to answer because it's so subjective. The best advice I can give you is to check out some titles from some of the most popular VN artists:
Popular artists with cute moe art:
- Kobuichi & Muririn (Yuzusoft) - Moe art with highly dynamic sprites
- Nishimata Aoi (Navel) - Very popular moe art
- Hatozuki Tsumiki & Mitsuki & Yakuri (Eushully) - High quality fantasy manga art more on the moe-side
- Koku (Purple Software) - Good combination of moe and erotic art
Popular artists with more mature characters:
- Komori Kei (Ricotta) - Very good erotic art with more mature characters
- Iizuki Tasuku (Hibiki Works) - Famous for the Lovely & Cation titles. Very good erotic art with more mature characters
- Amasaka Takashi & Kaneusu Tato & Kazuma (Clip Craft) - Very good erotic art with more mature characters
- G Yuusuke (Light) - Specialized in very stylish chuuni art
- Kagami (Lilith) - Infamous artist specialized in erotic fetish art with more mature characters
Popular artists with a more realistic art style:
- Suigina Miki (Innocent Grey) - Very good mixture of realistic art with manga art and more mature and realistic looking characters
- Sei Shoujo (Empress) - Infamous artist specialized in erotic fetish art with more mature and realistic looking characters
- M&M (Astronauts: Sirius) - Very good erotic art with more mature and realistic looking characters
The question about the production values is somewhat easier to answer since it allows a more objective point of view. In general, titles with high production values usually have the following characteristics:
- Lots of event CG's
- Lots of character sprites - not only for main characters but also for side characters
- Even background actors are drawn (Very rare nowadays, but older titles like Clannad, Shuffle or Ever17 put a lot of effort into those)
- All characters voiced, including high profile voice actors
- Backgrounds are drawn and not cheaply produced blured photographs
- Usage of high profile artists
- Usage of special effects like animation
- High profile writers and composers
Titles with huge art content are hard to realize with just a single artist. For that reason titles with high production values usually have multiple artists, or one artists who gets several years time for it, like Komori Kei with Walkure Romanze or Sei Shoujo with Starless. Background art in particular can also be bought by an external company without having to bother your main artist with it.
Since older titles usually have rather old fashioned art styles by todays standards, I'll split the answer in older and newer titles.
Amongst older titles, I'd say the highest production values probably had MuvLuv Alternative, followed by Fate/Stay Night and Ever17.
If it comes to newer titles, the top contenders are probably Eushully titles, because they also have a fairly high quality J-RPG included. Minori is famous for expensive high quality art but they usually have few characters and I'm not sure if their newer titles stand out as much as their older titles. I personally liked the character designs of their older titles ef, Eden or Supipara more than their newer titles. The Koihime series from BaseSon is also infamous for their high production values because their titles are extremely long with millions of voiced characters. MangaGamer still couldn't afford to bring over any of their newer titles. Innocent Grey titles also have very high production values even if they seem to be a lot cheaper than BaseSon titles. Otherwise MangaGamer wouldn't have been able to afford them - cheaper voice costs I guess.
If I should make a personal pick of a title I played with the best combination of high production values and good looking art the it would be probably Kamidori Alchemy Meister from Eushully. The content was just insane and I really like their art style. In my subjective view, there are two companies which have an even better character art style - Ricotta and Clipcraft - but neither of those can deal with the pure mass of content of an Eushully title.

Gotta be Key.