A Couple of builds led by a Solid HQ Choice are emerging.
Tau are masters of shooting, Kroot, Firewarriors all have the ability to pour out torrents of fire to lay into infantry so I’ll skip that. I want to talk about the Tau’s new weakness, heavy armor and heavy (AV 12 flyers). Yes the masters of Landraider-slaying are no longer.
1) HQ with Broadside Detachment.
This is a great combo unit against flyers and medium to low armor.
Take a full broadside team with the high yield missiles and interceptor.
Take an HQ with Iridium Armor and the Puretide Chip and missile as well.
Also take a full complement of Missile drones.
This means when a flyers, Drop Wolves, etc comes in you can shoot 16 twinlinked missiles at them. Sure you need a 6 to hit, but it’s twinlinked and you can reroll failed pens.
2) The Assassin with Passion Shadowsun!
Shadowsun is even better in 6e.
While Tau can get Melta from Piranhas, Crisis Suits or Riptides, you have to be close enough to make it count. Shadowsun can infiltrate. In fact all Stealth Suits can infiltrate. Also, they can take Target Lock.
As deployment zones are not that huge armor tends to clump up.
Take a maximum Stealth Squad with 2 Melta Guns and Target Locks (200) points. Join Shadowsun (with her C&C drone of course). You start 18 inches away unless you can grab some great cover so it’s not a true melta shot, but you can probably side armor shot 3-4 targets, with a reroll from the drone. Additionally those Strength 5 shots of which you have 20 are excellent for laying into troops.
For added lols take the Serg/Vre upgrade on a Burst Cannon with the markerlight and target lock..
Paint another armored target. In your Fast Attack take a Piranha with 2 Seeker Missiles who can move 30 inches and use the Markerlight to fire it.