Elise, Kha'Zix, or Rengar

Don't play any of them in a solo lane, at least not in ranked. Elise is probably the best out of the 3, but I'd say Rengar is the most fun, especially when ahead.

Elise is pretty powerful in ranked/normals for an AP bruiser. Rengar is a cheese champ, so jungle is best for him. As long as you're good with him you can go ranked and climb pretty high. Kha.....well...he's been smacked kind of hard but slowly being healed over time. But his damage is pretty stupid when alone. All 3 champion are really fun and has a big variety of difficulties and play style. Get each one if possible and try each one in each lane. If you like it enough then you can play it. [slayer-pantheon-thumbs]

Kha'zix is weak right now but metas change. Disregard his current state because I promise you he will be strong once more in time. I would go with Kha'zix simply because of his versatility. Kha'zix is typically a jungler but throw him into a solo lane (top or mid) and he functions just as well. He has mobility, sustain, wave clear and is a great one on one duelist. In team fights try and position yourself in such a way that you can pick off anyone who strays too far from their team. And he's not that complex to play so if you drink and play this game he isn't a champion you have to put on the "I'm drunk so I cannot play this champion anymore" list. Elise is the hardest to play. She's kind of... she's weird. Yes she is AP but she functions best as an AP bruiser who prioritizes AP pen rather than straight AP. She's nimble, she's deceptively tanky and she's able to pierce through defenses both MR and health stackers. She will take time to get into a carrying position as her early game is kind of bland. She's not awful early game like say Warwick, but she needs items to actually become useful to her team. Out of the three champions listed she will be the hardest to learn. Rengar is a cheese champion as previously mentioned. All you have to do on him is master his instagib combo and you are set. He's got good clear times, decent mobility and a diversified kit where proper management of his ferocity stacks opens up a diversified application of all his skills. He has hard and soft solo target CC, he has stealth, he has the highest burst in game (alongside LeBlanc). He's kind of useless though if he does not get kills and damage early on.