What is a better test of skill nowadays?

I’m not just talking about the settings and gametypes, but rather the kind of people still playing the playlists?

Is Arena (top onyx-level) or MLG more challenging nowadays? Say, with a full team of 4.

Probably MLG, that gametype is only for those with experience. The Arena i 've seen alot of noobs playing there and a lot of pros playing there.

Both as they both have completely different setting and maps and well almost everything

But probably MLG if its vs. a team of 4

Most likely MLG, but only if its 4v4. And even then you may get unbalanced teams.

Outside of a 4v4 there isnt really any way to gauge skill in this game. The lack of a (working) ranking system just means whoever gets lucky and doesnt have abysmal teammates wins. Or the team of 4 vs team of 4 individuals gets wins constantly. Reach really is terrible on producing a fair game.

Arena for actually matching players based on skill.

MLG for everything else.