Before the rise of Synchro and XYZ monsters fundamentally changed the way Yu-Gi-Oh! was played, the main powerful monsters that players would summon were Fusion monsters. By using Polymerization, or one of its many variations, players could easily change the tide of the duel in their favor, substituting a handful of mediocre monsters for a much more powerful one.
Not all Fusion monsters are created equal, however, with many not being worth the card investment it takes to bring them out, while others are so bad that they aren't even worth the effort to cheat onto the field with the likes of Magical Scientist.
10 Dark Flare Knight
Some newest iterations of classic Fusion monsters summoned by Yugi Moto have been OK at best, but Dark Flare Knight just isn't worth the trouble it takes to bring it out. For starters, it's weaker than Dark Magician, one of the materials needed to summon it, and its effect doesn't justify it having such a low attack.
Secondly, it's a Fusion monster that requires another Fusion monster to summon, meaning it takes five cards just to bring this thing out. While it does have a secondary effect to summon a Mirage Knight when it's destroyed, Mirage Knight isn't that great either as it can only last through one battle before banishing itself.
9 Phantasm Emperor Trilojig
On paper, this enormous monster seems like it is worth the trouble. A monster with 4000 ATK is nothing to scoff at, but it has no inherent protection effects to save it from very common cards that can easily remove it from the field before it is able to do anything.
Its main burn effect is also incredibly bad considering it takes three level 10 monsters and Polymerization to summon it, when Superdreadnaught Rail Cannon Gustav Max takes only two level 10 monsters, doesn't need a Fusion Spell, and has a more consistent damaging effect. Even in a dedicated Sacred Beast deck, there are far better cards to summon from the Extra Deck than Phantasm Emperor Trilojig.
8 Gem-Knight Aquamarine
Gem-Knights have had rises and fall in popularity over the years, though their peak was easily with Gem-Knight Fusion and Gem-Knight Seraphinite, both of which were responsible for improving the already great Performage Performapal deck and putting many cards on the Forbidden and Limited List. Gem-Knight Aquamarine, on the other hand, seems more like a bad Elemental HERO Fusion.
Its ability to change into defense position after attack seems good at first, but its appalling 1400 ATK means that it's not going to do much damage even if it does manage to attack. Pure Gem-Knight decks aren't exactly game-breaking, but Aquamarine is bad even by Gem-Knight's low standards.
7 All Neos Contact Fusions
Contact Fusion decks were all the rage before the introduction of Synchro monsters, with the likes of Gladiator Beasts easily being one of the best decks of its generation. The Neos contact Fusions, however, are a completely different story. Aside from them all needing to have Elemental HERO Neos on the field to summon, a level seven monster with no effect and requiring two tributes to summon to the field, the majority of Neos Fusions will return to the Extra Deck at the end of the turn they are summoned.
There have since been cards added to the game that help mitigate this effect, such as Neos Fusion and Neo Space, but that isn't enough to make any of these Fusion monsters particularly good.
6 Darkfire Dragon
One of the first-ever Fusion cards to be released in the Legend of Blue-Eyes White Dragon booster pack was this little gem. While it has some of the better card art of its time, the monsters that are required to summon Darkfire Dragon aren't ones that should ever be used, as they are both terrible Normal monsters with low ATK and DEF stats.
This could have been fine if Darkfire Dragon had decent stats, but even the majority of other Main Deck monsters from this booster can easily attack over a monster that requires using three cards to summon. It's not the worst Fusion monster in existence, but it's definitely bad.
5 Sanwitch
On the surface, Sanwitch isn't as bad as it appears. Having a way to send Sangan and Witch of the Black Forest to the Graveyard on the controlling player's terms is a good thing until you realize that they both need to be on the field for their effects to activate. This means the only way to get both of their search effects during the Fusion summon process is to have one of them survive for a turn, which is very unlikely to happen considering their poor ATK and DEF stats.
To make matters worse, Sanwitch doesn't have an effect of any kind and only has semi-decent ATK and DEF stats on its own. It is an OK target for Fusion Weapon, but that isn't even a great Equip Spell when compared to the likes of Axe of Dispair and Mage Power.
4 Musician King
Another Fusion monster that requires Witch of the Black Forest, this card would have probably been forgotten by time if it hadn't featured in the Yu-Gi-Oh! anime episode where Yami and Tea went on a date. Like Sanwitch, Musician King can potentially activate Witch of the Black Forest's effect during the summoning process, but it also suffers from not having an effect like Sanwitch.
It also doesn't help that Lady of Faith isn't a card that should be in any players' deck, as there are many far better Spellcaster monsters that can work alongside Witch of the Black Forest.
3 Giltia The D. Knight
Joey Wheeler had many terrible Fusion monsters in his deck, though none were as bad as Giltia the D. Knight. Even Flame Swordsman, which has weaker stats overall, was a better Fusion monster when he couldn't bring out Thousand Dragon, as its combo with Salamandra often helped keep him from losing.
The anime must have noticed how bad this card was as a Fusion monster, as it was changed to be a Normal monster in the Battle City tournament and was how the audience learned about tribute summoning.
2 Mavelus
Originally exclusive to a handful of video games before finally having a wider TCG release in 2014, Mavelus puts even Darkfire Dragon to shame with how bad a Fusion monster it is. Unlike Darkfire Dragon, one of Mavelus' materials, Tyhone, could be theoretically useful as it is a level four Normal Winged-Beast, making it a decent target for Rescue Rabbit, but Sky Scout is just better in every way for the same purpose.
Its second material, Wings of Wicked Flame, is about as good as the two materials needed for Darkfire Dragon's summon, so there is no reason to use Mavelus for anything other than a coaster.
1 Fusionist
When it comes to bad Fusion monsters, there is no card in existence as bad as Fusionist. Not only is the card artwork pretty bad, even by early Yu-Gi-Oh! standards, but the two monsters that are used to summon it have better combined ATK and DEF than Fusionist.
It does have some very niche usefulness as a non-Tuner monster that can be summoned with Instant Fusion to Synchro summon Naturia Beast or Naturia Barkion, two Synchro monsters that were very powerful during the peak of X-Saber decks, it's nowhere near enough to justify taking up valuable Extra Deck space.