Year 1991 was the first with monthly issues and for that reason the hardest. Let's first try to determine its prominent features for VNs:

- Number of adult games increased greately to some 100 titles overall

- PC-98 era arrives with mouse support and the older systems finally lose support

- RPG percentage remains high at some 20% while ADV grows significantly to  some 50%

- New sub-genres appear - point-and-click, quiz, board game.

- Some games start to use action icons instead of command selection thanks to mouse use.

- It's the last year without any government regulation. After November 1991 shoplifting incident regulations being created.

1.  
2. Masterpieces of 1991:
   1. CAL II
   2. Cosmic Psycho
   3. Cybernetic Hi-School Part 4 ~Ape Hunter J~
   4. ELLE
   5. Gidyy
   6. Jesus II
   7. Nike
   8. Nostalgia 1907
   9. Psychic Detective Series Vol. 4: Orgel
   10. Psychic Detective Series Vol. 5: Nightmare
   11. Rance III - Leazas Kanraku -
3.  

Those were the games with great stories and a professional touch. And I'm biased here towards ELLE since it's the only one that got renewed version in 2000s. Otherwise it might be a serious competition.