There are many similar threads to this one, and the solution(s) to your question has/have been widely discussed many times recently.
The ABSOLUTE best keychain light out there right now, IMHO, is the IlluminaTi. It's made of titanium, runs off of a AAA-sized cell, uses an XP-G R5 emitter, and puts out about 115 lumens (stated.) It's a bout 55 dollars off of Battery Junction, prior to CPF discounts.
The best bang for the buck? iTP A3 EOS 3-mode or the Maratac AAA. They're the exact same light, except the Maratac has more aggressive knurling, and the iTP has a beefier keyring attachment point. About 20 to 25 bucks anywhere.
Most suited to carrying on a keychain full of keys? iTP A3 EOS Stainless Steel Edition. 30-ish bucks anywhere.