I would say it depends on what you're actually doing. If you're grinding monthly quests solos is better as you have a easier job doing stuff (not applicable for kills , but is for finals)
Duos and quads are amazing for friends, you get tons of kills on quads however I'd only recommend it with friends. With randoms you lose extremely easily.
Trios are the perfect balance for grinding out statistics, so this could help if that's what you're aiming for.
Overall try all of them out with friends and with fill squads and decide for yourself! The fun is the most important factor here.