I have found that the monster books Kobold Press released for 5e have a wider variety of interesting and challenging abilitues, while staying within the 5e philosophy. They have been very useful for spicing up my 5e game. A lot of them seem a bit on the tough side for their CR, but maybe we've just gotten too complacent with less scary 5e critters.
The books are The Creature Codex and Tome of Beasts. They're massive, minimal fluff, but full of great, flavourful monsters. I don't usually get too excited about bestiaries, but these are good.
They ARE very good, but when you look at the hit points and damage output for some of these CRs, compared to the core monsters, they're almost unfairly deadly, so be wary about using some of them. In general though, these books are the equivalent of an AD&D MM2. Great artwork and great writing.
For example, here's a CR 9 insect, and it's attack stats are pretty scary:
ACTIONS
Multiattack. The horakh makes two claw attacks and one bite attack.
Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 22 (4d8 + 4) slashing damage. If the bite attack hits a target that’s grappled by the horakh, the target must make a successful DC 16 Dexterity saving throw or
one of its eyes is bitten out. A creature with just one remaining
eye has disadvantage on ranged attack rolls and on Wisdom
(Perception) checks that rely on sight. If both (or all) eyes are
lost, the target is blinded. The regenerate spell and comparable
magic can restore lost eyes. Also see Implant Egg, below.
Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target.
Hit: 17 (3d8 + 4) piercing damage. If both attacks hit the
same Medium or smaller target in a single turn, the target is
grappled (escape DC 14).
So, you're looking at 10d8+12 damage per round - not including Crits! - and potentially permanent blindness.