I consider myself to only be good at fusions PvP wise. As such, I have used all of these fusions except Pocket Infinity in PvP before, and want to see what Bnet has to say on what I consider to be the best fusions in the game.
I see the Thesan as the best of all the fusions in the game. It can kill in five bolts and has very well rounded stats overall, with a clear focus in stability. Its stability is not quite enough for it to reach 100 with injection mould, so Braced Frame is the better option. Its very high equip speed combined with hot swap makes it the best fusion for using the best fusion rifle strategy - precharging in the hip and letting loose the burst the instant you ADS. This significantly improves recoil and makes landing 5 bolts incredibly easy.
The Saladin's is often considered by players who don't know what makes a fusion rifle great as the best fusion ever introduced. I disagree. This is the second best fusion rifle ever. Compared to the Thesan, it has slower charge time, higher damage per bolt, higher range, significantly worse (BASE) stability, worse reload (who cares) and - this is what makes it second best - much lower equip speed. The higher bolt damage is the main draw of the weapon, making it easier to kill because only four bolts need to hit. However, every other stat is essentially worse or negligibly better than the thesan, meaning you are sacrificing the charge and equip speed of the Thesan for needing to hit one less bolt. However, if you are playing correctly it is rarely needed to have the four bolt kill.
The Vacancy is a straight downgrade to the Thesan due to its only advantage being a 1 point increase in damage per bolt, making the damage 49 per bolt. This does not change the amount of bolts needed, so it is nothing comparatively. It is a good substitute until you get a Thesan, however, because it is still a very good fusion rifle.
Plan C has its exotic perk allowing for very fast kills, making it certainly an S tier fusion - if it had better stability. Also, the exotic slot could be used better for something like SUROS or NLB.
Panta Rhei is an average fusion apart from its recoil direction, which is a very high 80 from the get-go, making it very easy to control. It is best used for the tactic of aiming for the feet. However, it is still best to use the ADS trick with the thesan and go for center mass.
Pocket infinity is an abysmally bad fusion...when there isn't more than one person in the room. If there is, enjoy the triple down you put no effort into getting.
Other is for...other things.
Don't vote for other if your favourite fusion is the Vex Mythoclast; it is only a fusion rifle for the purposes of how the bolts work.
[spoiler]sorry for the long post[/spoiler]
[spoiler]prepare for self bumping[/spoiler]
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Anything that falls under the slow charging-hard hitting archetype. After that, whatever has the best perks in my collection.