I found the secret room at the crooked crane, but was overwhelmed by the lich behind the door. Wot is the best way to overcome him? Is the prize worth it? Who should i have with me?

The prize is worth it (understatement of the year award nominee 2001).

The usual tactics for liches are always good. But if you want to be cheesy then just run in with a theif hasted up. Loot all the coffins and boxes etc then run back out again. If you are quick he won't get a spell off and he won't follow you for some reason if you only do it once. Or get all your fighters ready around the door and try the above several times. Eventually he will come out to face you and you can (hopefully) pulverise him after the protections have worn off. Jus remember to keep low HP chars out of the way as Fireballs and Pit fiends will cream them in no time.

What is the goody you get?

<Font color=green> I would run in, cast a few spells, and run back out to sleep. I did that until he was out of spells and I kicked his lily ass!

What i find to be the best tactic is a variation of the_loki_man 's -
Haste ur party, prtect them from evil.
Then run in there, wait for the Liches time stop to wear off, then run out and sleep, then rahaste ur party and go back in there- the pit fiend is unsumoned, the meteor swarms have ended, and the liches protection spells r gone, so just attck it.
the loot u get from this room is a ring (of animal contral i think), a sunblade (called daystar. +2, +4 vs. evil creatures, double damage vs. undead. Once per day can cast sunray- does alot of damage to everyone in the area exept ur party- undead must save vs. wand or be destroyed. Proficency type: longsword), A wand of frost, alot of gold (i think), and possibly a few gems.