Which team won the RLCS Season 1 World Championship?

The RLCS Season 1 World Championship was won by iBUYPOWER Cosmic.

The inaugural Rocket League Championship Series was announced by Psyonix in March 2016, as the developer began organizing a formal competitive circuit around the rapidly growing game. Season 1 culminated in a LAN final in Los Angeles on August 6–7, 2016, with a $55,000 prize pool.

iBUYPOWER Cosmic defeated European favorites FlipSid3 Tactics in the championship match. The victory made the North American roster the first official RLCS world champion. The team was later signed by G2 Esports, so historical records may connect the champions with G2 rather than their original iBUYPOWER Cosmic name.

The other options are later champions or prominent teams: FlipSid3 Tactics won Season 2, Cloud9 won Season 6, and NRG Esports won later RLCS world championships. That makes iBUYPOWER Cosmic the single canonical answer for Season 1.