One of the oldest champions in the game, both in his release date and lore-wise, is Zilean. His kit has remained unchanged for years, and even still remains relevant to this day.
Whether you're playing him in the Support role and helping your ADC, or in the mid lane and looking for plays with your jungle, Zilean is a League of Legends pick that will have your teammates love you for it, while also making you climb easily.
Zilean's passive ability allows him to "store" exp when he is near minions, and then use it to boost his ally and himself to the next level, once they've reached close enough. Doing this a couple of times allows you to gain an exp advantage over the enemy, and every level in League of Legends gives more stats than items that can be bought for a kill's worth of gold.
His Q is an incredible harassing tool and a herding one. Since his bombs stay on the ground, you can use them as a deterrent allowing your ADC to farm, or to encourage the enemy mid-laner to move towards the side where the gank is coming from.
His E ability is one of the most broken abilities in the game. It is, once you've put some ability points into it, a point and click root for all intents and purposes. Coupled with Zilean's W, this allows you to slow enemies, stun them, and speed up your allies in one spell rotation. Add in some situational Mythic items such as Everfrost of Shurelya's, and you will be stopping your enemies in your tracks, and enabling your allies.
His Ultimate is, basically, a Guardian Angel that does not take up an inventory slot, and can be applied to any member of your team. With you on the team, your hyper carry will have the game of their life with nothing to fear, and you will carry the game from the shadows.