These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Early game, High Speed and Attack makes Spit Fire lethal, especially against single bosses who only have moves with long charge times. While not necessarily a high damaging move, the low MP cost coupled with high speed allows you to stun lock many foes.
Ice Statue: near-instantaneous casting time, high damage, and has a chance of causing paralysis to boot. If anyone asks why Megadramon's so popular, that's likely a factor.
Megalo Spark is basically the air-elemental version of Ice Statue.
Full Power/Full Potential. It raises all your stats by large amounts. In fact, the reason the Bonus Boss is so hard is because he starts the battle by spamming it! Megadramon can use it too.
Bug. A spell that throws three bugs at your enemy, and they climb on them and explode. Because they can explode in random intervals, one can explode, only for your opponent to get back up and get blown up by ANOTHER.
Those techniques are nothing compared to the Monzaemon suit in Toy Town. If you get a Numemon, you can interact with the suit to evolve Numemon into a Monzaemon instantly. You can now breeze through most of the game with ease with your brand-new Ultimate digimon, who can also learn Megalo Spark.
Good Bad Bugs: The glitch where your partner's name at the startup screen would end with many extra "mon"s, Mojymon dancing on the water, cloning items via the shop in the Gear Savannah, making the doors vanish in Toy Town with the DG Dimension move...
For whatever reason, you can keep feeding Digitamamon's stat-boosting meals to your Digimon partner even after he isn't hungry anymore. If you have a lot of money to spare, you can exploit this glitch to easily max out your Digimon's stats.
Guide Dang It: From the guide itself; the official game guide goes into no detail whatsoever on the latter part of the Grey Lord's Mansion sidequest, except that it allows you to recruit SkullGreymon.
Misblamed: Many of the glitches in the game are a result of the English localization team.
Most Annoying Sound: If your Digimon digivolves into Whamon, you will learn to hate the sound it makes when it floats after you. Additionally, the sound it makes when pooping is best left undescribed.
Vademon has a constant semi-mechanical blare from his UFO, which and be headache-inducing.
In-training Digimon have a tendency to use their cries multiple times when you're standing in one spot for a long time. This can be especially annoying if you're doing something such as fishing, and they won't shut up.
Tear Jerker: When your Digimon dies, especially if it's your fault for not bringing enough items or not grinding its stats enough.
That One Boss: The absurdly strong Greymon that ambushes you outside Jijimon's house.
This gets even worse if you don't know its coming and check your prosperity rating while you have a Baby or In-Training Digimon. The battle is activated when Jijimon tells you that the prosperity rating is equal to or greater than 15. Good luck taking on Greymon with your Punimon, as there's no chance to get away from the battle once its activated.
They're not too difficult if you're prepared; the thing is, as mentioned above, that unless you read about it somewhere beforehand, you'd never know they were coming, and thusly probably wouldn't be prepared at all.
Birdramon is still rather annoying. She comes with the ability Prominence Beam, and can do incredibly large amounts of damage, especially on weaker Digimon. She becomes easier if the player chooses quick attacks to interrupt it.
Garurumon is pretty strong thanks to his Ice Statue. To make it worse, he will ask you for a rematch where you are unable to give commands to your partner.
Piximon. To clarify, he only appears randomly on the screen, along with two other bosses, and you must walk in and out until you finally see him. Since he's right next to the entrance of Tropical Jungle, anyone that sees him can just pass him off as another boss, especially if you're not familiar with the fact that Piximon is an Ultimate level Digimon. He also proves to be a Pintsized Powerhouse, having 5000 health, powerful stats, an attack that causes confusion, and Bug, a skill that can actually hit multiple times. It's no wonder this guy has caused many deaths for many players.
That One Sidequest: Getting Monochromon means having to sell enough goods to get a certain amount of bits. This one is entirely a Luck-Based Mission, and seeing as how you're meant to overprice certain items to gain more profit, it's hard to tell when a Digimon will be fine with it, and others, like Weedmon, who will complain about everything, even lowering the price.
Penguinmon's Curling Mini-game is a bit of a challenge, and can take a couple tries before you beat him. If you don't, prepare to have to fish, because the only way to rematch him is to give him some, even after he is recruited. To make matters worse, there's a medal you can earn that's focused on getting a perfect score (10-0) in the game, which is much harder to do. Have fun.