The What
Variance lowest to highest>
CG>SnG>MTT
The Why
CG never pressures you on a bad run to play any of the
hands or to get too pot committed with a mediocre hand against a loose raiser. Even if you do lose a deep-stacked pot, you can top-off your stack and be deep stacked once again to make good plays and win back what you lost and more with optimal +EV decision making.
MTT is worse than SnG as there's no limit on how many people can join (and a huge variety of when in the tournament they can join depending on rebuy or late-entry etc) Additionally, MTT blind structures are very often the equivalent of at least a Turbo SnG. Since even the structure, table size, player pool and everything is unknown on top of the obvious source of variance (cards) MTTs are higher variance form of tournament than SnGs..
Oh yeah, even the prizes in MTTs is down to variance (more people = bigger prize if buyins pass what the GTD prize pool expected) the bigger the amount of people also the more places actually get prize-money. So, even the PRIZE you get will be down to variance.
The win-rates
Naturally CGs give the best winrates, on top of the lowest variance you can play the most CGs at once vs any other kind of game (ok actually this is physically untrue with
pokerstars as they offer unlimited tournaments at once but limit CG tables to 24)... What I mean is mentally you can cope far easier as a pro CG grinder playing 15 at once than a MTT pro can even cope with 3+ if they make it to the mid-game of all 3. This is because there's so much to take into account.
All of that makes CG the highest winrate format of the game for a pro grinder.
MTTs despite being higher variance than SnGs seem to attract far more fish-per-player than SnGs. Since SnGs are more reg-infested and both are fundamentally tournaments (meaning you are forced to play with your opponents, can't walk out of a table, can't back down no matter how bad you run) then it actually means MTT grinders make more than SnG grinders just because they have more chance of being vs bad players.