MOdern Horizons II

Overall, this set is a cubelet builder's dream. There are so many cards with tons of unique play that almost any card from the set is viable for a cubelet. If you are new to the format or getting tired of figuring out which cuts to make to include new cards, this set has enough to serve as a core for your cubelet. The list provided in this post are those that are just a cut above the rest and would be the ones to seriously include if you are updating a "Legacy" level cubelet.

Esper Sentinel
Annoying to use a removal spell for can gain you some much needed card advantage.

Out of Time
Has the potential to create an exceptionally chaotic board, which is always an exciting element to cubelet.

Prismatic Ending
Fairly safe removal spell that will get most permanents in a cubelet, and the cost is fair as it scales.

Subtlety
More than likely, the creature/planeswalker will end up on the bottom of the library, which is interesting for blue in a cubelet.

Bone Shards
Planeswalkers are always difficult to put into a cubelet, but this gives one more answer to make them viable.

Damn
Being able to be a spot removal or wrath may open up one card slot in the 100 with this flexible option.

Ragavan, Nimble Pilferer
It's monke, what more can I say that hasn't already been said.

Endurance
A unique way to manage the shared graveyard.

Jade Avenger
Bushido is an interesting mechanic that doesn't see a whole lot of play, but this "bear" could see some good play.

Dakkon, Shadow Slayer
Surveil is powerful, the minus is fair and the ultimate isn't overpowering if you built your cubelet correctly. Great planeswalker to include if that is your thing.

Geyadrone Dihada
Similar power and potential as Dakkon. Demands attention from the field. Fair inclusion.

Master of Death
Interesting interactions with the field and surveil is a strong mechanic.