best drain cleaner bathroom/shower sink?

Mr Muscle, but keep air circulating if using it indoors.

HYPOCHLORITE

As strong as you can get. Buy a sprayer and go mad.spray and walk away,come back in 15 mins.

Drives,Walls etc.

Get a tub of Sodium hydroxide(NaOH), mix some into water until dissolved and then pour it into the drain. Be careful with it though, it gets hot(reaction) and it will burn you even when it has cooled down !

Thick bleach.

Pour some down and leave it for a few minutes.

If it's really blocked, close your eyes, and when you're feeling lucky press down with your plunger. Nothing works better than suction.

Washing Powder if nothing in the house, with boiling water from kettle spout

Spirit of salts. Be sure to wear gloves and protect your eyes.

I'm not a serial killer

Dennis Nilsen recommended ... 'Cannibal' Drain Pipe Cleaner

boiling water if it's cooking  Fat.......or unscrew the U bend

Good luck

I have used my car jet wash down the shower waste before, that worked well, and chemical free

caustic soda
(Sodium hydroxide- as mentioned by somebody else)

Caustic Soda cleared my bog when nothing else would.

Suprised there is no advice here from Culvin.

Dennis Nilsen recommended ... 'Cannibal' Drain Pipe Cleaner

Wasn't he the one who got caught when he called in Dyno-Rod to unblock his drains?

Go to your nearest plumbers merchant and ask them for industrial strength acid drain cleaner, it is 96% sulphuric acid. Make sure you read the instructions and follow them exactly as stated.

What exactly is blocked and why? I'm not a big fan of pouring a heap of really nasty chemicals in that are just going to be end up in the sea or river.

A proper and inexpensive plunger is fantastic for unblocking a sink quickly, you just need to know to block the overflow up so the suction really gets down where it's supposed to go and isn't just working the overflow hole. Works on a shower as well.

I did buy some strng chemical unblocker for something and it came with a long saw tooth flexible plastic snake tool that you could stick down and through all the piping to give it an extra good rasp if something chunky was blocking it. Might have been Mr Muscle, it was branded anyway. You can buy flexible unblock/snake tools separately, you just push them down and then rattle them around, reusable.