I actually think the prevalence of day/night cards will increase in casual play, but not tournament play. Basic meta-gaming: you don't want your deck to fall apart simply based on random matchups. If more day/night will be available, the higher the possibility that you could face your deck's bane. And therefore won't risk it. A splash of day/night is still very possible though. My problem with Syndicate's fickleness toward time of day is that it is meant to be strong in both, when it really just makes them weaker and more conditional overall.
Prior to Terror in Venice, it was extremely rare.
I've been in 4 (going on 5 this weekend) Call of Cthulhu tournaments, including a pair of well-attended Regional Championships (19 and 14 players, respectively). As the T.O. of each tournament, I keep all the deck lists after the events to write up reports, etc... so far I've seen over 50 decks meant for competitive play. In all of those events, I've encountered only 1 deck that included the Day mechanic as a cornerstone element.
Why? I think there are 2 main reasons. First, the Dreamlands cycle has been out of print for some time making it hard for newer players to have access to any Day/Night cards at all. Second, before TiV there weren't that many Day/Night cards to choose from. However, TiV has given Day/Night a big shot in the arm. When we finally see the Dreamlands reprints, I think it will show up even more.
And to get this back on topic:
My top 10 nominations are Intimidate (SoA), Tear Gas (TTftS) and Patsy (Core).
I like Intimidate better than Forcing the Truth because it costs only 1. Sure, it's not as fancy as Forcing the Truth or even Low Blow (Core), but it's cheap and works great with Syndicate rush. When it comes to events, I'm hard pressed to pay more than 1 unless I'm nuking an opponent's character, especially with a deck that needs to be firing off as many characters as possible in the opening rounds.
As for Tear Gas, it plays nicely with Fixer (SoA) for a turn 1 cost-reduction, and is a great way to destroy annoying invulnerable characters like Carl Stanford (SoA) or Fthaggua (TKatG) that can slow you down early on.
Last up, the Patsy. I can't believe this guy isn't even on the honorable mention list. Combine with B. Ramsdale Brown (TKatG), Corrupted Midwife (ER) or Hungry Dark Young (Core) to really annoy your opponent.
For honorable mentions, what about Tattoo Artist (MoE) or Street Tough (TbtA)?
Regarding Patsy, I'm willing to put it on honorable mentions, but as a card in itself (without recursion elements) it's really rather sub-par imo. It's only good as a character if there are no defending characters to oppose you, otherwise it will go insane. It's not the end of the world because you'll likely just sac it to remove tokens next turn, but I'd rather have characters on the board with more staying power to push through stories, not delay my opponent (whose tokens will go away anyway when I win the story).