I’ll just give you a full-overview of what I take usually:
Level 1 : Right Helix
" In Addition to slowing enemies. Temporal Distortion now hastens allies. "
(Allows me to quickly get to positions and to run away from any assassins coming straight at me, also allows my teammates to catch-up to enemy battleborns If I place it in the middle, which I usually do for minion-wave-clear)
Level 2 : Mutation
"Predatory Strike owls automatically reveal nearby targets."
(I feel like this is one of the most overpowered mutations that Marquis can get, allows you to see players through walls at times if used effectively and If you put your owls in places where stealth assassins HAVE to pass through, you can also counter their invisibility.
Usually players turn invisible when they take damage, therefore what I do is take few shots at a battleborn that can go invisible, I place my owl in-front of me and wait for them to go invisible just so I can either track them or kill them).
Level 3 : Mutation
"Increases Bindlebane’s magazine size."
+2 Clip Size "
(I feel like it’s a mandatory mutation if you want to get kills, can’t tell you how many times I had to use the extra 2 bullets…)
Oh and I don’t like neither of the helix’s at level 3, because recoil = useless IMO, and extra zoom on scope is really hard to use and get’s in the way.
Level 4 : Left Helix
"All enemies inside a Temporal Distortion bubble take damage over time.
+41 Damage Per Second"
(I use this always for minion wave clear and to line up shots on slowed enemies that get caught in the bubble)
Level 5 : Left Helix
"Eins, Zwei, Die’s bonus damage is triggered on the second consecutive hit of a marked target, rather than the third."
(The only reason I like this is because of my ridiculous attack-speed on items + level 7, allows me to quickly bring down targets rather than burst them down, also allows me to have more chance for error as I don’t need to be super accurate to deal full burst damage).
((I would choose the Right helix for 25% bonus damage on third shot, only if you’re trying to go full burst damage and plan on hitting 3 shots in a row on important targets))
Level 6 : Right Helix
"Increases Temporal Distortion’s casting range. +30% Cast Distance"
(I think I take this passive always, because it allows me to stay safe at the ledge/stairs and I can just continue to slow/clear minions now from a distance without having to shoot at them (wasting more time)- try to shoot minions only if you don’t see anything else to shoot at, thralls or elite bots or enemy players. Clearing minions is not always our job.
Level 7 : Left Helix
"Increase’s reload and attack-speed.
+25% Reload and Attack-speed"
(This + items, it’s possible to get a bunch of attack-speed and turn your sniper rifle into a semi-automatic rifle rather than bolt ^.^… it’s what allows me to bring down players REALLY FAST)
((Because of almost no recoil on the actual sniper itself, you can stack up attack-speed and still be accurate, I do take the right helix only when I feel like 2-shotting their squishies by critting them in the face, or by dealing massive damage to their front line.))
Level 8 : Left Helix
"Upon detonation, Predatory Strike owls leave behind areas of effect that deals damage over time.
+720 Damage over 6 seconds."
(I’ve gotten so much kills that I couldn’t get to, because of this passive!) 
Level 9 : Right Helix
"Intensifies the slowing effect of Temporal Distortion, further reducing enemy movement speed
30% Slow"
(This is basically me, maxing out my Q {Time Bubble} to be a slowing monster that allows us to land shots more effectively at medium range, also we can use this to zone enemy battleborns from passing through a certain area of the map, I use it to block off tunnels on Incursion).
Level 10 : Right Helix
"Bindleblast’s shots can now pierce world terrain to strike targets. Enemy Battleborn are revealed in the scope."
(Mwahahahahah! I love this one, a combination of this helix + the owl mutation at level 2 is DEADLY, I’ve managed to snipe players down from my base into their base because of it… though that was a somewhat lucky shot, I basically shouted in the microphone “Thorn! Move out of the way!” as the ult flies past Thorn and obliterates the enemy battleborn. While the Thorn player hilariously reacted to it with total confusion as to what just happened!
“That player ofc was @seraphoenix”
Hopefully this list of helix’s I personally love taking, will somewhat help you out and decide how to play Marquis.
Oh and as a bonus 
here is a list of my Items I use frequently on Marquis!
Loadout 1
Item 1:
Wristguard;
+8.72% Attack Damage
+4.98% Attack Damage for 3 seconds after taking health damage.
(I don’t have any better wristguards 
)
Item 2:
Wristblade;
+7.87% Attackspeed
+4.49% Attackspeed for 60 seconds after killing a player or a major enemy.
Item 3:
Implant;
+9.97% Critical hit Damage
+6.64% Critical Hit Damage for 60 seconds after killing a player or major enemy.
Situational Items I have in other loadouts:
+14.44% Recoil
+8.66% Shield Penetration
Stack +3.47% Shield Penetration for each hit on an enemy within a short time. Max 5 stacks.
+9.57% Critical Hit Damage
-12.76% Recoil
Marquis Only- Critical hits reduce the active cooldown on Bindleblast by 2 seconds.
+19.98% Reload Speed
+9.18% Reload Speed while health is full
(This items is nice with the 25% reload speed we get from level 7 helix)
Items are bound to be improved sometime, I know my 1st item is not what I want.
