Hard to say as it depends on where you are, which rivers you're targeting and what species you're after. Out west where I live its trout in cold, cold water, so I use the following flies for success:

KG?s South Fork Special


Hook: DaiRiki 125 size #14
Thread: 8/0 Olive/Dun
Bead: Copper 3/32
Tail: 8-10 Pheasant-tail fibers
Rib: Chartreuse Uni wire ? small
Abdomen: Ice Dub ? pearl
Thorax: Ice Dub ? olive brown
Wingcase: Pheasant-tail fibers
Wing/Legs: Olive Hen Feather

KG?s Deep Purple Peril


Hook: DaiRiki 125 size #14
Thread: 8/0 Black
Abdomen: Black Thread
Rib: Holographic Purple Mylar ? small
(Two coats of Sally Hansen's Hard As Nails)
Thorax: Black dubbing
Wing: 2mm white foam

Improved Shop Vac


Hook: DaiRiki 125 size #14
Thread: 8/0 Black
Bead: Gold 3/32
Abdomen: 3-4 Pheasant-tail fibers
Rib: Chartreuse Uni wire - small
Thorax: Peacock
Wing: SAAP High Vis - white

Rainbow Warrior


Hook: DaiRiki 125 size #16
Thread: 8/0 Red
Bead: Silver 3/32
Tail: Brown Saddle Hackle Fibers
Rib: Silver Uni wire ? small
Abdomen: Pearl Krystal Flash
Thorax: Ice Dub ? pearl, red-hue
Flashback: UTC Mirage - opal, large

"Big Ugly" Rubberlegs:


Those are five of my favorites during winter, fished dead-drift, under an indicator.

Kelly