Mystic Knight Series[]
The Mystic Knights don't excel in HP, ATK, or RCV, but their real strength lies in their active skills of "Changing another color's orbs into its own color orbs". These skills provide a constant (and potentially explosive) source of damage for mono-color teams, and have a short cooldown time to boot. You can't go wrong with sticking 1-2 of these knights in a mono-color team.
After evolving into their 5-star forms, 
Ancient Dragon Knight and 
Chaos Dragon Knight become Dragon-types, and are good Leaders for a early-to-mid-level player's Dragon Team. After gaining Ultimate Evolutions, the Mystic Knights have all gained a Balanced subtype, further increasing their usefulness in mono-Type teams. 
Ancient Dragon Knight, Zeal fits well in particular with 
Noble Wolf King Hero, Cu Chulainn.
Once a player reaches mid-level or beyond, these monsters can be farmed from a number of event dungeons, so they are not recommended as a starting pull. Most of the Mystic Knights even have special survey dungeons made for them, which will inevitably roll around again.
for a 2/4/2 Balanced team, but no one is going to do that.
, but Water Balanced types are a bit harder to come by. Not popular as a leader, but can be used if you don't have .
, 
, or 
light teams. Not useful as a leader, but you could team up with 
...but who's doing that?
to form a 2/4/2 Dragon team. Dragon types have low RCV, though, so it might still not be enough of a multiplier.