I am an ordinary layman who spent an unacceptably long time on this character on this role.
my pick is diamond

-adc on toplane kinda weakearly and can lose teams prio on map but it also good option for late game or late mid game.
- Aphelios have enough versality to stay toplane for scale bcs of toplane xp gain and opportunity to farm 10cs/min with platings even if go 0/10 u can stabily scale for late/mid game
-nobody know how to play properly a 1v1s a lot master tier players just by playing my game

-one of the best late adcs
-can hard beat lane oponent
-versality of builds
-can 1sht some topers early on with lethality and Crescendum, the Chakram
-can poke to the tilt much of people
-you can be easily underestimated
-can turn a lot early 2v1s

-ur adc lol
-can be weak if fall behind depends on situation
-usually will be babysitted
-your teammates won't ever trust you
-hard to make any preasure on sidelines in some games so u nedd group but its can be hard
-psychosis
-mental instability

Tier 1
Berserker's Greaves huge attack speed allows to kite best dps option from boots
Kraken Slayer - best crit item rn broken scaling insainly good in almost any 1v1
Stormrazor best after kraken good for any 1v1 but deals less
Lord Dominik's Regards best pen. item bcs of aphelios hp scaling
Rapid Firecannon strong
Galeforce stronger when ie usualy

Tier 2

Immortal Shieldbow decent instead bt
Plated Steelcaps sustain option
Boots of Swiftness decent option instead Berserker's Greaves
Blade of the Ruined King strong 1v1 item allows to kite slow enemies give nise sustain and have decent build path
Infinity Edge strond crit item slow to build doesnt give attack speed good only for 3rd slot sometimes 2nd
Black Cleaver underrated item gives crazy armor pen.
Mortal Reminder strond item less viable then ldr
Maw of Malmortius still very strond item even after nerfs can replace
Bloodthirster strong dps option
Phantom Dancer decent 3rd slot if u builded ie 2nd but rfc rn stronger imho
Trinity Force maybe

Tier 3
Guinsoo's Rageblade strong item but doesnt fit aphelios kit but viable
Wit's End maw better but in on hit build viable
Titanic Hydra good sustain/dps option but worse when others good with Guinsoo's Rageblade
Guardian Angel optional
Randuin's Omen strond def item
Death's Dance better when Guardian Angel if u have enough sustain
Runaan's Hurricane for teamfights

tier 4
The Collector if ahead viable
Statikk Shiv garbage
Force of Nature never build this
Mercury's Treads dont build

only viable runes

Precision
Most common rune page which allows to scale throughout the game
Overheal best choice which good with Shield Bash and overall aphelios setup
Triumph common rune take if u do not want play with Overheal

Legend: Alacrity good choice for early but Legend: Bloodline usually better
Legend: Bloodline best choice good sustain + hp when fullcharged

Cut Down good against any high hp enemy with Lord Dominik's Regards
Last Stand best rune good with urs Exhaust Immortal Shieldbow Barrier Maw of Malmortius

Sorcery
Second best rune page for aphelios
Nimbus Cloak strong option against engage
Absolute Focus good scale oprion with ur Overheal
[[gathering storm] best scale option but very long keep it in mind

Resolve
survivablity options
Shield Bash very good with Overheal
Conditioning common scale rune gude if ur laner is afk
Bone Plating most usual and common rune from whole tree best on laning and any hard 1sht trade
Overgrowth

Domination
good scale rune page
Taste of Blood optimal sustain option especially good if u dont using Legend: Bloodline
Ghost Poro its just Absolute Focus 2.0. Much better when Eyeball Collection bcs its easier to stack and it gives vision = op
Treasure Hunter common option good for snowball
Ingenious Hunter strong option good with Immortal Shieldbow Blade of the Ruined King Guardian Angel etc.

a little afterword First Strike from Inspiration tree isnt good option. I tested it a lot even in afk farm lanes it wasnt good enough. Later on the game it can be usefull with any Calibrum, the Sniper Rifle or Moonlight Vigil play but i think other runes much better

ur best weapons on the game its Calibrum, the Sniper Rifle with Crescendum, the Chakram so u need keep it in mynd and set other weapons around it
usually im using something like this :
Severum, the Scythe Pistol>
Crescendum, the Chakram>
Calibrum, the Sniper Rifle>
Infernum, the Flamethrower>
Gravitum, the Gravity Cannon

Calibrum, the Sniper Rifle slow to use so be careful by combine it with other weapons, but best with Crescendum, the Chakram in medium range fights

Infernum, the Flamethrower q with Severum, the Scythe Pistol second can heal up ful bar hp from minion waves use it to bait enemys

Crescendum, the Chakram often useless in lategame so use Crescendum, the Chakram turrets for selfpeeling

Severum, the Scythe Pistol strongest aphelios weapon allows you to play the game. With decent attackspeed its almost doesnt have any animation its ignores yasuo and samira wind wall, it strong combine with other weapons, his q in late game can heal up like 1k hp. be sure to use is it properly

Gravitum, the Gravity Cannon fcks up half of assassins in game use it with with severum to stay alive against scary guys

Severum, the Scythe Pistol
-you can do one auto before Severum, the Scythe Pistol q its nothing new, but u can deal another fast auto if u auto ur enemy before ur Severum, the Scythe Pistol q ends

Calibrum, the Sniper Rifle
- Calibrum, the Sniper Rifle Qs can fck up ur attack speed so try to dont use it in melee range.
so how to auto reset it - u need decent amount of attack speed like Berserker's Greaves+w max and use ur Calibrum, the Sniper Rifle q on range of ur auto and instantly auto ur target so u can deal 1 more auto before it lands and after proc mark with zero delay

Infernum, the Flamethrower
-also nothing new auto-q-auto

Crescendum, the Chakram
-U cant reset ur Crescendum, the Chakram autos manually u need to have Calibrum, the Sniper Rifle and abuse medium range with turrets and mark from Calibrum, the Sniper Rifle allows u to deal high Crescendum, the Chakram dmg from much higher distance without losing dps
Gravitum, the Gravity Cannon
-I didn't find anything but small tip u can q Gravitum, the Gravity Cannon befora auto lands

u need to teach what can do other champs and now when u can be aggressive
u can win lvl just by cheesing from bush and dening minions aggro by bush leaving enemy without any exp

aphelios insainly strong i has ended a lot games just with 3 items by my self