Evaluation is based on my Fast+9 runs, and is highly subjective.

Evaluation weighs heavily towards their early-game performance and their potential to deal with crowded mobs later on. In most cases, if you survive the early game, you will win the game.

One great example is Flicker/Flash/Sun. As fusion bases, Flicker >= Flash > Sun. But early-game damage Sun>Flash>Flicker. Picking Sun early helps handle overcrowded mobs while picking Flash does not help much, and picking Flicker early almost always leads to a failure. In middle-to-late game, whether you choose Sun/Flash/Flicker does not matter much as they all good enough to deal with mobs, with the help of later picks. Therefore, my opinion is Sun S tier, Flash A tier, and Flicker B tier.

Second example is Holy Laser and all Beams. They are powerful in the entire game. All laser evolutions are good early picks, and are powerful when fused with Maggot/shotgun/etc and ghost/wind/etc. Holy Laser is just the best among them. Therefore I rank Holy Laser S tier, and all others A tier.

Another example is Overgrowth/Hemorrhage/Frozen Fire. When fused with Sun/Flash/Flicker, they almost always solve the mob problem. However, they are useless before being fused. When it comes to mob clear, Overgrowth is the best and Hemorrhage is the worst (sometimes Hemorrhage x Sun/Flash/Flicker is not enough to clear mobs). Despite Hemorrhage stands out in most boss fights, I will still give it a B, and give Overgrowth/Frozen Fire an A.

C tier consists of "good-to-have" but not essential evolutions, and having them or having their fusions alone often does not lead to a win. D tier evolutions are just BAD, I tend to skip them in my Fast+9 runs. Most C&D tier can be replaced by other high-tier evolutions (or even basic balls).

Some of my runs are posted on youtube (Check the playlist Ball x Pit: Fast+9):
https://www.youtube.com/watch?v=jy34GMXgkRc&list=PLr_3z76aDsBE9WqLTx-FkxUagl2JwVoEl

Tier S: Holy Laser, Sun

Tier A: All Beams, Maggot, Bomb/Nuclear Bomb, Frozon Flame, Overgrowth, Flash

Tier B: Hemorrhage, Flicker, Shotgun

Tier C: Leech, Sacrifice, Spider Queen, Mosquito King, Voluptuous Egg Sac, Magma, Inferno, Phantom, Sacrifice, Swamp, Glacier, Sandstorm, Mosquito Swarm, Blizzard, Lighting Rod, Storm

Tier D: Vampire Lord, Virus, Berserk, Succubus, Incubus, Lovestruck, Assassin, Black Hole, Nixious, Wrath, Soul Sucker, Satan, Nosferatu

1 Maggot x Holy Laser/Nuclear Bomb

2 Flicker/Flash/Sun x Overgrowth/Hemorrhage/Frozon Flame

3 Ghost/Noxious/Sandstorm x Holy Laser

4 Nuclear Bomb x Dark

The balls can be classified as either Trigger or DMG. Ideally, we want to fuse a Trigger ball and a DMG ball, to hopefully trigger DMG ball effect as many times as possible.

Below I list some common infusion bases and their type. Bear in mind that this is not the complete list, and I may well not test and update the list in the future.

Trigger type
T1 Trigger ALL: Maggot, Mosquito Swarm, Mosquito King, Voluptuous Egg Sac, Spider Queen, Shotgun
T2 Trigger Status Only: Blizzard, Storm, Inferno, Noxious, Sandstorm
T3 Full-screen Trigger Status only: Sun, Flash, Flicker

Strictly speaking, there are only two TRUE Full-screen triggers. One is Flicker and the other is Nuclear Bomb. We classify Sun and Flash into Full-screen Trigger as they perform similar to full-screen trigger in practice. Nuclear Bomb is often regarded as DMG rather than Trigger, and is omitted here. It is worth noting that Nuclear Bomb can trigger full-screen Black Hole but Flicker cannot.

DMG type
D1 Triggered by hit: Holy Laser, All Beams, Bomb, Nuclear Bomb, Lighting Rod
D2 Triggered when launch: Swamp, Magma, Glacier
D3 DOT: Overgrowth, Frozen Flame, Hemorrhage, Phantom, Inferno, Virus, Leech

Laser evolutions can also be classified as T2 Trigger. For instance, when paired with DOT anything hit by the laser is afflicted DOT. But Laser evolutions are just better as DMG source and we list them in DMG.

TLDR
Overpowered combination:
T1 x D1
T3 x D3

Okay combination:
T1 x D2
Noxious/Sandstorm x Holy Laser/All Beams: Not recommended as Ghost/Wind x Holy Laser/All Beams works just similar

Meh:
T2/T3 x D1