While they aren't for every deck or game, I generally enjoy:
Mana Cache - Lots of interesting choices. Keep mana back for answers and risk "giving" mana to the opponent. Tap out, even with nothing to cast, and potentially waste mana to prevent giving mana away
Storm Cauldron - More choices, but this time its what you need now vs. what you can "save" for later.
Teferi's Puzzle Box - This one is all about variety. Some people hate it because they can't "plan," but I find it interesting when you have to adapt each turn's new hand.
Teferi's Realm - Usually interesting in the way things phase in and out around the table. Though, often, one payer will just keep picking enchantment over and over to try keeping the realm phased out. Where's the fun in that?
Gravity Well - For something so simple, it's funny how many player's plans are thwarted when they can't attack with flying. Can often change combat dynamics in weird was.
Phyrexian Splicer - Another sleeper that seems simple on the surface, but the possibilities with mix and match abilities make combat math so fun. Note: Neither the creature losing nor gaining the ability have to belong to you...