The best build is Multi-Melta and Heavy Flamer.
Some people claim that 2x Multi-Melta or 2x Heavy Flamer are better, but in actual fact it is incorrect.
You want to be moving the Land Speeder 12" each turn to get within 12" for 2D6-AP, or in flame range. Plus the added benefit of opponents needing 6's to hit you in close combat.
Due to the need to move as far as possible each turn, you are only able to fire 1 weapon as neither of them are defensive weapons, and thus making 2 of the same weapon pointless.
So then why take both a MM and HF if you can only fire one at a time?....
Simple answer.... Versatility.
There are arguments that the Heavy Bolter / Typhoon Missile Launcher (or even Assault Cannon) build is effective too. While i do agree that it is effective, it is also a LOT of points to invest in such a fragile chassis.
I believe that a DakkaPred is much better value for points compared to the DakkaSpeeder, as it is much more survivable.