All of these are just my opinion but some of my favourite mids to have on my team are-

Kassadin and Leblanc - Both are mobile enough to follow up Renekton's engages and can usually either help him focus someone down or exploit the chaos of him diving into a fight.

Orianna - Long range, good wave clear, good sustained damage, shield speed and ulti are insane with Renek.

Kennen - garbage waveclear, but otherwise has some good mid matchups and insane teamfights with Renekton. Lots of damage, CC and speed, hard to pick off, strong engage and counter-engage.

Katarina - good at following up with Renekton, very good at cleaning up fights and executing the people you soften up, you can start fights by eating CC and making them run out of options to deal with Kat

Fizz - like LB/Kass only with less range and a better engage

Ahri Ekko Vlad and Lissandra also work very well with Renekton