Demonic Spiders: Dang, these Yellow and Pink bloons move too fast! Oh, hey, a rainbow bloon... AH! That was close... And why's this brown ceramic thing taking so many hits? Whoa, look at the size of this one. What does M.O.A.B. stand for, again? (*psst* it stands for "hope you've got enough cannons")
In BTD2, it was the black bloons who fit in here- they were the only bloons to be immune to cannons, and since the strongest bloon was the rainbow bloon, cannons were used a lot. The only option against a frozen black bloon were the upgraded boomerangs, which fired in a rather strange pattern. Later installments gave the cannons shrapnel to deal with black bloons.
From BTD5... Regenerators... Regenerators... REGENERATORS... Note that this is a "trait", which means that any bloon that isn't a MOAB-class can have it. It isn't so bad for weaker Bloons, but for stronger Bloons that have more than one children, both children will regrow back into the original Bloon if left too long. For example, if a Regrow Rainbow gets popped, you end up with two Regrow Zebra Bloons, both of which will regrow, leaving you with two Regrow Rainbow Bloons
Camo bloons in BTD5. In BTD4, Camo Bloons were a single type of bloon (like blacks and whites), and towers could accidentally hit them by targeting a bloon near them, so if one in a stream of them was popped, it could start a chain reaction. In BTD5, Camo Bloons are more like a "trait", similar to Regenerators. Every bloon they contain is a camo bloon in itself. Add that to the fact that only a few towers can spot camos, and well, prepare to lose a lot of lives.
And later, you get Camo Ceramic Bloons. Enjoy! Thank goodness you don't face regenerating Camo Ceramic Bloons during main levels.
What's that? Playing on Hard? Well, one of the last five levels involves a rush of BFBs and MOABs...and it starts off with a rush of camo regen bloons. Fun!
Depending on how early you buy them, Super Monkeys and Banana Farms. Also, this game on high levels can be considered a PCU breaker when there's too much happening onscreen. Minimize your window to hear things at their regular speed.
Practically all 4th tier upgrades in BTD5 count, in various ways too. Even among them, the Temple of the Monkey God stands out. It is the upgrade for the SunGod, but it fires a single stream of sun beams instead of 3 rapid streams of sun rays. Where it gets interesting, however, it its sacrificing feature. When it is upgrade, it will sacrifice any towers in its range and absorb their powers* Its secondary powers consist of golden missles, glue splatters, ice blasts, tornadoes and a choice between a 360 spread of darts or blades. A max Temple of the Monkey God essentially becomes a rapid-firing nuke with a concentrated explosion. Not even Z.O.M.G.s will last long against it!
Another big example is the Super-Wide Funnel upgrade for the Bloonchipper. It can destroy a MOAB after two attacks, and can bring a ZOMG down to BFBs very easily.
There's a premium upgrade in the fourth and fifth games called "Double cash", which doubles how much money you begin with, and how much you get from the bloons. There's also a premium upgrade called "Healthy Bananas", which allows Banana farms to give you more lives - buying both will give you enough money to buy a banana farm from the beginning, with a little extra to buy one more cheap tower. Once the farm gets a few upgrades, it becomes ridiculously easy to rack up staggering amounts of money and lives.
In Bloons TD Battles, MOABs and BFBs become this when you play in Assault Mode. You can send a MOAB onto your opponent's track starting at round 18, which is long before they're likely to be able to deal with one, even if they have unlocked the better towers and upgrades. If they don't realize they can do the same to you (or if they don't have the money to do so)... near-instant win!
Hypersonic Tack Shooters. In the first two games, by upgrading via the tab key, you could boost upgrades way beyond their normal capability. Most of them, however, didn't work. One exception, though, was the fire rate of a Tack Shooter. Using the tab key, a player can create one that fires as fast as a Super Monkey! This was fixed by BTD 3.
While not exactly a "bug", it is possible to set up a strategy that allows players to repeatedly pop regenerator bloons (of at least black/white level) for an infinite amount of time. To do this, place a 3-0 Monkey Apprentice & 1-4 Glue Gunners in a location where the wizard can blow the bloons to the start & the gunner can slow them down. The thing is, it'll require a good defense to keep the bloons from overwhelming the player (a Ray of Doom placed so that it covers most of the exit will suffice, as will a Quadruple Spiked Mines set up if the bloons in question are rainbow bloons). Unfortunately, on the flash version, it's been nerfed as of the update that introduced 3 new maps, but it's still possible to gain unlimited money with the use of the Supply Drop ability.
Memetic Badass: The Super Monkey is an elite monkey if there ever was one. He fires faster than a machine gun & can switch to laser blasts when the going gets tough...& then plasma vision when it gets even tougher. He is capable of targeting enemies from far away & can gain even more targeting range. As of the 5th Tower Defense in the Bloons series, he can turn either into a Physical God that can pummel bloons when given the right sacrifices, or a Humongous Mecha that is more than capable of holding its own against massive armies.
Sailor Earth: The most common blimp suggested to the dev team is something that just tanks damage and spawns a number of Z.O.M.G.s upon being popped.
That One Level: Quite a few, as most of the Expert/Extreme difficulty ones from 4 and 5 qualify. However, we'll list the main ones.
The Bloontonium Lab has 3 very short paths that form the hazard symbol. This track also doesn't have any water on it, so Monkey Buccaneers are out of the question unless you get a Portable Lake.
Monkey Vs. Bloons is a remake of the pool levels from Plants vs. Zombies. As a result, it has 6 paths that go straight to the house without any bending or turning except at the end. At least this track has water for Monkey Buccaneers.
The Triangle of Insanity takes the form of Penrose Triangle. It has 3 entrances that double as exits.
Another example from 4 would be Ant Hill track. It has one entrance, but the bloons go around a loop near that entrance before heading down one of 4 separate exit paths. It has 2 puddles of water, but they only go between the 1st, 2nd and 3rd paths, none between the 3rd and 4th paths.
Clock's an extremely awkward track to play on, as the path changes as the clock's hands move, which changes every round.
Special mention goes to the downright sadistic Down The Drain (with four short paths). And god forbid you play on Impoppable Difficulty... The only thing that can save it is the fact you can do Save Scumming because it's an Expert track, rather then Extreme.
Wish to achieve 100% Completion? Get ready to cry; Tar Pits has five separate paths, and the bloons will reverse direction every 2 rounds. To boot, it's also Extreme, so you can't even save the game at all.
Crypt Keeper has 1 enterance... But by the end, it splits into 10 separate exits to defend, the most exits seen in the series. Very arguable, but this can be Inverted if you play in reverse mode, where you have 10 entrances and 1 exit.
Yet again, the Mobile/Steam version, with Double Double Cross. It has FOUR entrances and exits, each being very short. To rub salt in the wounds, The entrances are also the exits, so don't even think of using Spike factories. It's not the hardest expert map, but it definitely stands out as one of the worst.
There are many other levels that could count, but the ones above are probably the most obvious examples.
Averted on the Cool Math Games version of the first game, and as of BTD2 and beyond... Save for Dummied Out in the Flash version for BTD3, where data for placeholder text for tower descriptions does indeed use a swear, as can be seen here.