The answer to your question depends on what your current champ pool consists of.
To be a well rounded player with experience in all roles and positions you must know at least one champion for each position/Role.
So without knowing what champs you already know and own I cannot provide a direct answer, instead I'll outline a few key things about each champion you have in mind...
Ahri - Is a medium to high hard champ to master, she performs as a ranged ap assassin, She's different from regular assassins in the sense that she has a sustain ability, CC, and an escape. A rare combination on a champion with a jump and high damage capabilities. She is played mid, pick her up if you don't have a mid ap assassin. Although hard to master, well worth learning and incorporating into your scope of play.
Cassiopeia - She is a hard champ to master, If played correctly she can do outstanding in lane and become very threatening. She uses a combo that relies on properly predicting your opponents movement and countering with an aggressive two-step combo. She is a traditional mage in the sense that she doesn't have an escape, and can be a big threat in team fights. She excels in 1v1 combat and does well in aoe situations as well. A very hard champion to play but once mastered has few realistic counters. pick her up if you dont have a mage mid.
Diana - She is a mellee ap assasin that combines both high damaging burst spells with attack speed and a touch of survivability. Diana can be played in the jungle, top or mid. Since she is melee she has a disadvantage in the early game until she is level 6. Pick her up if you don't have an ap assasin jungler.
Ekko - Ekko is another mellee ap assasin. He is currently considered to be over powered, with most likely get nerfed, and is perma banned. You wont get to practice him much in ranked due to the banning, and in normals he will be picked almost every game on both sides of the team. I would recommend not picking him up since diana can fill the same role to a certain degree and at least you can actually play her right now. That being said, ekko has a jump, a stun, a skill shot ranged nuke, a slow, a passive that does damage every 3 hits, and a heal, and another jump. he has the most robust kit ever seen on a champ in leauge history. Hes extremly hard to master since he has so many options when it comes to battle. I would learn him once hes off the radar
Vladamir - Vlad is a bruiser mage with tons of suvivability and an aoe debuff. He is often played top but can also go mid in some instances. He scales to the late game much like a nasus or swain. Pick him up if you dont have any beefy ap champs (note hes not a tank since he doesnt have any stuns or slows inherently)
Blade has a decent guide about vladimir so he may be a little biased with opinion :P
But Vlad is not a bad choice at the moment. Also the Ekko hype will die down once he gets nerfed. When it comes to the other champions you mentioned I have no strong opinions about who is better :(
The answer to your question depends on what your current champ pool consists of.
To be a well rounded player with experience in all roles and positions you must know at least one champion for each position/Role.
So without knowing what champs you already know and own I cannot provide a direct answer, instead I'll outline a few key things about each champion you have in mind...
Ahri - Is a medium to high hard champ to master, she performs as a ranged ap assassin, She's different from regular assassins in the sense that she has a sustain ability, CC, and an escape. A rare combination on a champion with a jump and high damage capabilities. She is played mid, pick her up if you don't have a mid ap assassin. Although hard to master, well worth learning and incorporating into your scope of play.
Cassiopeia - She is a hard champ to master, If played correctly she can do outstanding in lane and become very threatening. She uses a combo that relies on properly predicting your opponents movement and countering with an aggressive two-step combo. She is a traditional mage in the sense that she doesn't have an escape, and can be a big threat in team fights. She excels in 1v1 combat and does well in aoe situations as well. A very hard champion to play but once mastered has few realistic counters. pick her up if you dont have a mage mid.
Diana - She is a mellee ap assasin that combines both high damaging burst spells with attack speed and a touch of survivability. Diana can be played in the jungle, top or mid. Since she is melee she has a disadvantage in the early game until she is level 6. Pick her up if you don't have an ap assasin jungler.
Ekko - Ekko is another mellee ap assasin. He is currently considered to be over powered, with most likely get nerfed, and is perma banned. You wont get to practice him much in ranked due to the banning, and in normals he will be picked almost every game on both sides of the team. I would recommend not picking him up since diana can fill the same role to a certain degree and at least you can actually play her right now. That being said, ekko has a jump, a stun, a skill shot ranged nuke, a slow, a passive that does damage every 3 hits, and a heal, and another jump. he has the most robust kit ever seen on a champ in leauge history. Hes extremly hard to master since he has so many options when it comes to battle. I would learn him once hes off the radar
Vladamir - Vlad is a bruiser mage with tons of suvivability and an aoe debuff. He is often played top but can also go mid in some instances. He scales to the late game much like a nasus or swain. Pick him up if you dont have any beefy ap champs (note hes not a tank since he doesnt have any stuns or slows inherently)
Personally, I'd go for Ahri yet every single champion you mention is overall great.
But Vlad is not a bad choice at the moment. Also the Ekko hype will die down once he gets nerfed. When it comes to the other champions you mentioned I have no strong opinions about who is better :(
Edit
I did some extra thinking and here is the order that I would personally get them in Vladimir Ahri Ekko Diana Cassiopeia