Sub zero.
Josh and I fished a spring creek in -11 and had a wonderful day. I love watching big spring down in the ATW and its giant weed mats during sub zero. The way it steams in a bright morning sun is breathtaking.
What sucks is when your waders break and you get frost nip ;-)
You just gotta know your limit, take tons of breaks, move slow and take time to warm up in your car every 30 mins to and hour.
It's not for everyone, but like scotch it's an acquired taste I've come to enjoy.
It's funny this came up. My soon to be wife asked me the other day if I was ready for spring and I looked down and said no. In shock she asked why not? I told her we never really got that deep freeze that I love to go out in. Every time we got the little bit we did, I had kids or obligations and we didn't have enough of them that I had free time. She said yeah I had a feeling you would say that. This woman understands me very well. She likens me to a gypsy. I love nature and everything that comes with each season. There is a vulnerability, a quiet solitude and a beauty to a deep freeze. It allows you to truly appreciate how lucky you are to be alive and have the things you have living in this country. I've grown to love it but only two rules appliy, You must come very prepared and you must except it's not a fishing trip but rather a nature trip. Catching a fish is a bonus, the true spirit of it lies more in the realm of watching gods plan at work.