I recommend Moersch's MT3. And, as dasBlute said, it's extremely toxic... I'd also recommend toning outside if you can. It appears to be a very heavy non-odorless vapor, that hangs in the air even after the fan has been turned off and you don't realize it until you get symptoms. Keep your unexposed film and papers out of the vicinity as well, as I have noticed chemical fogging on both, from toning inside the d/r (with fan on). There are threads here on that.

I really enjoy MT3 sepia toning but do take precautions. Even suited up and masked I got a serious sore throat from vapors lingering even after a week. I have seen really beautiful prints made from it though so don't let it stop you.

Good luck! You'll really enjoy the results.