This is based on the opinion of the user Zendarmanitan and isn't official.
This is an article about my 10 favourite fusion monsters with reasons.
Cyber Twin Dragon This card has 2800 attack points and can attack twice! With Power Bond, that's two attacks of 5200! This is quite a realiable card and not very difficult to summon.
Ultimate Gear Golem This monster has a high attack and an awesome effect. It is quite easy to special summon for Ancient Gear decks.
Cyber End Dragon This card has 4000 attack and can inflict piercing damage. With Power Bnd, it's power doubles to 8,000 which is quite difficult to defeat although you would lose 4000 lifepoints later.
Evil Hero Lightning Golem This card can destroy any of the opponents cards once per turn. This makes it very useful although its attack isn't enormous.
Chimeratech Fortress Dragon This card gains 1000 for each mnster used and the opponents can also be used as long as they are machine. DNA surgery can be used to make every face-up card usable as long as you have a Cyber Dragon.
Chimeratech Overdragon This card gains 800 for each monster used and can attack a multiple of times. Overload Fusion or Power Bond can be used for a massive efect. Power Bond doubles it minimum 1600 to 3200 and it can even reach the 10,000s! Overload fusion is good if there are many dragons on the field or the graveyard.
Evil Hero Dark Gaia This card can be fused with up to 7500 attack! Also, since it is fusion summoned with Dark Fusion, it cannot be targeted the first turn. All face down monsters your has are flipped in attack and flip effects don't work. This is a very dangerous Evil Hero and its materials can be easily milled with Reasoning, Monster Gate or Magical Merchant then Gaia can be special summoned with Dark Calling.
Elemental Hero Divine Neos This card is the final fusion monster Jaden Yuki uses. It has a powerful effect that allows it to gain 500 attack each turn and an effect until the end phase. This card can do massive damage when combined with the effect of Flare Neos as long as Neo Space is on the field.
Elemental Hero Magma Neos This fusion card has amazing attack potential since it gains 400 for every card on the field so it will automatically have 3400 attack points and with Neo Space, it will have 4300 attack points. If there are 5 cards in both spell and trap zones and 5 monsters in both monster zones as well as Neo Space, this card will have 11,900 attack points!
Rainbow Neos Rainbow neos is my favourite fusion monster for a variety of reasons. Not only does it have 4500 attack points but it also has many great effects that can counter many different decks. It can get rid of all the opponent's monsters if the opponent is using an offensive deck, it can get rid of all the opponents spells and traps so you are safe and it can get rid of the opponents graveyard if it has to.