Facing Tau and BA

Should I go serpent spam, gun boats (splinter racks) or Grotesques?

I guess it depends on your style. All those options describe small portion of your army. All are viable but you need to know what you are doing. :-) Obviously also depends on your opponent - are you facing mech force or what? Do you expect Flyers?

But TBH, I believe in non-tailored lists. Make an all-comer and stick with it no matter what you face. Only finetune it to suit your style.

Leadership works quite well against Tau. However, if your meta is full of marines everyone will know you tailored a list to fight the Tau. Do so at your own risk.

Tau are also very vulnerable to extreme MSU. Yes the ignore your cover, yes they kill whatever they shoot. Yes if you charge theme with anything the overwatch will kill you. But they can only kill a couple of units a turn in this way. Tau are much mor vulnerable to 6 units of incubi in venoms or units of Three reavers than they are to say a corpse thief claw or grots in raiders. You have to have a bunch of units that can all charge together on turn 2.

Blood angels are a different animal. They are extremely dangerous when they deep strike in but after that if you can keep your distance you can generally shoot them to death as they generally use shorter range weapons. Just watch out for flamers. I often deploy my warriors outside of their transports against BA so they can't hit both with one template.