Ekko has been a somewhat overlooked assassin in recent years. He saw the spotlight as a brusier toplane and later jungle, but his midlane assassin playstyle has always been a bit underestimated. In practice Midlane Ekko is actually quite powerful, and extremely versatile into many picks midlane. His strength lies in his roaming play-style. Much like Katarina or Talon, Ekko plays like a Roamer who can make high impact ganks to the side-lanes. Like Zed Ekko can dive easily and escape without much consequence, and like Kat Ekko is excellent at cleaning up teamfights. Unlike either he has one of the most powerful CC spells in the game. Ekko is a great all-around assassin. He scales well into late game, can split, teamfight, or 1v1 with the best of them. Perhaps his best trait is how well he is at countering other assassins. Because his ult acts as a panic button it's difficult to outright assassinate Ekko. He's weak long duration roots like morgs or Lux's, but those are relatively rare. Great if you enjoy the assassin playstyle, done want an all AD team, and hate Fizz with a passion.

Ekko, is built entirely around landing his phasedive empowered auto. All his other abilities revolve around this gap closer, and being able to quickly engage and disengage means the difference between a victorious and dead Ekko. I like to think of Ekko a bit like an Ahri: a Skirmisher Mage/Pseudo Assassin. His burst damage is stupid high, but he provides a lot more utility than a typical assassin. He's extremely slippery, and it often works to bait your enemies into diving you when you're actually one E away from freedom. I find that his W is often more useful for zoning than it is for actually stunning, but if you can catch them un-aware on a gank it can be devastating.

Protobelt is just too good on Ekko not to rush. Since landing E is so essnetial to Ekko it's hard to argue against this high damage gapcloser. Couple that with Ingenious Hunter's CD reduction and you'll be a nightmare to deal with. Lichbane is a no brainer, and for the most part this is a pretty standard Ekko Build. I think that if you're snowballing and roaming hard it can make sense to go the Lichbane > Ludens route, but it usually makes sense to rush Protobelt.

Ekko still does well as a semi-tanky assassin because of his passive so if you find yourself facing off against a lot of AD deadman's is still a surprisingly strong option, but I would only build it after lichbane. In a similar vein, the amount of CC built into Ekko's kit can make him work well with Liandry's. If you find yourself facing off against a tank heavy team Liandry's can be a strong substitute for Rab.

As I've said Ekko is played around his E so many of his combos are based around securing his E, or using his E to proc a stun.

E > Q > AA
The bread and butter Ekko combo. This does impressive damage, and procs his passive for a quick getaway. Use this if you want to trade, or if they're low enough to delete. Make sure to pay attention to the return path for your Q as it provides a huge chunk of damage (especially late game)

Q > E > AA
This is the reverse bread and butter. You use this combo to more easily set up your E. If you can land your Q square with their hit box it can be difficult for them to escape, and practically ensures a landed E. This is more useful early game when you lack MS or protobelt to force an E.

W > E > AA > Q
This is a powerful, but situational combo. When you want to set up a gank, or turn around a teamfight, this is a game-changer. The W stun is quite long (1.75 S) making it one of the most powerful AOE cc abilities in the game. Make sure to work that AA in between the E and Q so you can time the return of the Q with their exit from stun. This ensures that as they exit their stun they're already slowed and ready to get finished off.

Proto > E > Q > AA
After you pick up protobelt this will chunk squishies HARD. Proto > E > Q will likely bring them down to 50-60% health. If you land the AA you'll proc passive and bring them down to 25%. At this point they'll either sidestep the Q return, flash, or eat the 2nd part Q. If they eat the Q they'll be at <10% and you can finish them off easy with the execute on your W and the passive MS. If they sidestep they'll be even easier to run down and you can continuously proc the %W damage. If they flash, congrats you got their flash.

W > R > Q > E
This is extremely difficult to land, but can not-only oneshot, but it's a way to turn around even the most hopeless situations. This is useful when baiting and the combo works like this: They're chasing and you pop your W to slow their pursuit. As they pass into the W AoE you tap R reverse backwards on top and behind your pursuers, as you reverse through the W you'll proc the stun and nuke them with your R. They'll be chunked at this point and you'll be healed back up (if you were hurt). Drop your Q for the 2nd passive stack and E to proc your passive. They'll be either dead or close to it at this point. Throw in a protobelt at the end for good measure.

Ekko has excellent waveclear with his Q late game, but early on it can be difficult to shove the wave. Luckily your W passive really helps you last hit and makes farming under tower a reasonable option. Your Q is pretty bad at farming until you get rank 3 or so. For the early laning phase be careful with your Q since you can easily deny yourself farm with the awkward return time on the Q. Mostly focus on last hitting with your AA, and if they're punishing you just last hit with the Q. It often helps to trade a bit early since your early game fighting is actually quite strong. This should dissuade them from getting too close, and might give you space to farm properly.

Season 8 has a lot of changes for everyone and Ekko included. Domination looks like the best major rune path for now since it provides Pen and Burst, which as an assassin Ekko loves. Electrocute is the safe option since it mirrors TLD from the past mastery system. We take sudden impact because it's great with his highly mobile play-style. Eyeball collection provides some minor snowball potential. I like ingenious hunter because of Ekko's reliance on protobelt's CD, but Revenous Hunter is also good.

We go Sorcery second because I like its ability to scale him into later game. Since increased R uptime is so valuable on Ekko (for utility and playmaking) we take ultimate hat. And to scale him up into late game we take gathering storm. Precision is also an option since he's a melee assassin, but I like the R CDR on ultimate hat too much to take precision. If you prefer a slightly more aggressive build precision is very viable as well. With Triumph and Coup de Grace being very strong right now.