(http://wiki.dominionstrategy.com/images/thumb/5/51/Band_of_Misfits.jpg/200px-Band_of_Misfits.jpg)(http://wiki.dominionstrategy.com/images/thumb/5/5e/Overlord.jpg/200px-Overlord.jpg)
The ultimate "depends on the kingdom" cards. Also, the cards with some of the most annoying rule headaches.
For both Band of Misfits and Overlord, you're looking for Actions that are contextually powerful. What does that mean? Some Action cards are usually strong, and other Action cards are usually weak, but some cards are strong / weak depending on context. For example, Noble Brigand is strong if you know they have a Gold in their first 2 cards, and it's weak if no Silvers or Golds are on top of their deck.
Noble Brigand is an extreme example, because it goes between "+$1" and "+$1, opponent trashes a Gold, you gain a Gold", which is a huuuuge shift. However, the power of most Actions depends a bit on context. Consider a classical Village + Smithy deck. Village is contextually strong when your hand has a lot of Smithies and you only have 1 action. Smithy is contextually strong when you have plenty of actions. And, consider a +Buy card - strong when you have a lot of money, weak when you don't.
Band of Misfits and Overlord are especially powerful in games where you expect to run into several contexts where the best Action is different. This is because of an obvious observation: if you always play your Band of Misfits as the same Action, you could have just bought that Action. The only way you can get extra value is if you play your Band of Misfits as different Actions depending on the situation. For example, in a game from Dominion Championship 2017, I played Overlord as Rabble (when I needed draw), Village (when I needed the Actions), Explorer (when I had a Province in my hand), Chariot Race (when a previous Chariot Race revealed a low cost card on top), and Catapult (when I had good ammo for my Catapult). If I had wanted all of those effects, I could have bought an actual Explorer, or an actual Catapult, but it would have been much less consistent. I didn't want an Explorer or Catapult in my deck, because I don't always want to play an Explorer, or play a Catapult, and having the terminal when I don't want to play it is wasted space. But a card that could be an Explorer or a Catapult when I needed it, and a Village / Rabble when I didn't? Sign me up!
There are some trade-offs to this power. It costs more to buy BoM / Overlord, compared to buying the action directly. And, if the pile you want to copy is empty, you can't play BoM / Overlord as that action, which can strand your deck in some situations. In my experience, Band of Misfits is okay, but often has a lot of competition at the $5 cost spot. Overlord, on the other hand, is incredibly strong. Seriously, if you haven't tried it yet: just buy Overlord whenever you'd buy a $5 cost Action, or whenever you want to buy a $5 cost but don't have $5. 8 debt isn't that much more than $5, and the benefits are usually worth it.
Some final points: copying a Reserve card doesn't work, because once BoM / Overlord goes to the Tavern mat, it is no longer a copy of that Reserve card, so you can't call it anymore. Additionally, when Adventures tokens (+1 Card, +1 Action, +$1, +1 Buy) are on the board, BoM and Overlord get the bonus of both the tokens on their pile and the tokens on the Action they copy. If you have a lot of Overlords, consider placing your most important token directly on Overlords, instead of the Action you normally copy. Finally, for Conspirator, BoM / Overlord count as two separate Action plays, since you first play the BoM, then play the action it copies.