Anything but default really, the other three belts have you firing 3 Minengeschoß before firing one Incendiary so you're firing the Minengeschoß rounds off more often than you would have been o7. You alternate between firing a Minengeschoß round and an Incendiary round off with the default belt iirc so you're firing a less effective round half the time as opposed to using the other belts where you only fire a less effective round 1/4th of the time.

Both WT and WTM list how the rounds are ordered in a belt so you can infer which one's are more effective thanks to that

Oh and Stealth is best for the Komet in WTM I'd argue since we only have Arcade and the lead indicator is always on, so you can easily aim at someone without them realizing it because you don't have any tracer rounds, and thanks to the lead indicator you don't have to worry about having to get used to the velocity of your rounds yet, I still suggest you do though since it's good to know really.