Back in d days it was 3v3 bot games, however I think it got normalized so now the most efficient way is ARAMs because they don't take very long but also don't end too fast. You basically want games that last between 15 and 25 minutes I think because EXP gain slows down the longer a game takes.
Nexus Blitz might be secretly OP for leveling since it kind of fills the same nieche as ARAM but you don't meet those fucking ARAM accounts with all the good champions and matches WILL end pretty soon thanks to the walking nexus event.