1. MM creatures are fine, but most of them have trait that most DM do not use, Intelligence and Wisdom score. Lets take Orcs. Orcs have average wisdom, so they are sufficiently cunning to only fall for the same trick once. They will usually be led by a War Chief or Orogs or both. Those creatures have enough INT and WIS and battle experience to set early warning systems, adapt to tactics they have seen before, and overcome situations on the fly. They can communicate a fluid situation to their followers, orcs, and orcs have sufficient INT and experience to listen and follow directions. This means that when the OROG closes to combat range with lesser Orcs accompanying it the order will be given for one Orc to take the help action to give the Orog advantage on that first great axe attack.
Expanding on this, Hobgoblins have this trait right in the MM:
"Strategic Thinkers. Hobgoblins have a strong grasp
of tactics and discipline, and can carry out sophisticated
battle plans under the direction of a strategically
minded leader. However, they hate elves and attack
them first in battle over any other opponents, even if
doing so would be a tactical error."
That seems to me they will have a grasp of tactics at least as good standing field armies of the whatever kingdoms you adventure in. They will sentry everything, will fortify their camp every night while traveling, will conduct aggressive patrols on schedule, so they players will have a random encounter with a patrol unless they take steps to avoid one and cover their presence. If a patrol goes missing it will noticed, and measures will escalate. To me that means also that passive perception is out the window, the creatures will be making active perception rolls as they are patrolling.
Some creatures have low INT but reasonable to great WIS score, a Hydra (INT 2 WIS 10) wont have a thought out battle plan but is cunning enough to see who its greatest threat is. A Minotaur (INT6 WIS 16) is very cunning enough to retreat, track, ambush groups.
Creatures that lack both like Oozes are straight forward, however they are often controlled by others.
When you start getting more advanced creatures, like Giants, remember their group structure, and realize there is always a voice behind the throne type of advisor.
Really advanced creatures like Dragons don't get old and powerful by being dumb. They have been around for years, and have encountered many groups before that came to kill them. They will have rehearsed plans and courses of action that are effective. Things like doppelgangers will be disguised as something else and will have read your parties thoughts so they will almost never be surprised by any tactic used against them.
Another thing is to remember the characters are LEGENDARY, they are heroes! Word of their exploits will get around, so an intelligent creature will hear about them and be prepared. A super intelligent being with unlimited time and resources on their hands will know almost everything about the characters that is knowable. A lich or a vampire has nothing but time on their hands and cast multiple divination rituals repeatedly, they have little to fear from a Contact Other Plane spell also.