Which is, for you, the best equipment for a cleric/thief (SoA and ToB) ?

Quarterstaves, definitely a good quarterstaff. ;)

backstab is ok with quarterstaff, isn't it ?

Yes, it is.

Spoilerish

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shadow dragon scale armour OR
human flesh armour OR
robe of vecna (with UAI)
spare set of full plate when you get the bag of holding

staffs & clubs for back stabbing
mace of disruption +2
flail of ages
sling of seeking / sling of everard

boots of speed

For backstabbing get the Staff of Striking. Use the "sell then buy/steal it back from merchants" trick to recharge it when the charges get low. It will serve you well until you get the Staff of the Ram in TOB. Also make sure that you have the Cloak of Non-Detection to prevent True Sight spells from working while hiding in shadows. You can always switch it to a better cloak when you're in combat.

A weapon that can kill trolls is very important - Flail of Ages or Blackthorn.

If you don't mind cheese, I can tell you about a little trick that you can use to backstab while wearing heavy armor. Let me know & I'll reveal it here.

[ 08-04-2004, 10:40 PM: Message edited by: Desuma Malevois ]

Let me know the trick Desuma please...

This trick is simple. You sell an item to someone who accpets stealed items (Roger the fence in the Seweres), then steal it back. You get money (from selling the thing) and pay nothing to get a fully rechaged item back. (You steal it)

I think he was asking about the trick of backstabbing whilst in heavy armour Xen.

OK, this only works if combat has not commenced. Carry the heavy armor in your inventory. While hiding in shadows, sneak up behind your target and pause the game. Go to your inventory screen, change into the heavy armor, and go back to the game screen. You'll note that your character is still hidden in shadow. Attack your target and unpause the game. This works because, even though there are hostile figures around, you aren't considered to be in combat (and unable to change your armor) until the enemy has detected you.

Here's another nice backstabbing trick. If you wish to perform a backstab in a group of non-hostile beings, first have your auto-pause set for the death of your target. If your backstab kills your target, the game will pause and you have a chance to hide again before anyone else turns hostile. This works very nicely................[spoiler follows]................
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against the mage and Pooky in Mencar Pebblecrusher's gang.