And here is a rudimentary buyer's guide to flannel:
Low
Uniqlo - A very popular option on MFA that I'm sure you are all aware of. They make solidly warm, thick and well-fitting flannels for a relatively cheap price. Fit is slim.
Land's End - Great flannels, especially at sale price. Comparable to Uniqlo's in terms of quality and fabric. They fit well, if a bit more boxy and traditional than Uniqlo. Very good choices for heavier guys and athletic guys. They come in both a traditional and a tailored fit.
LL Bean - Soft, thick flannels. Like Land's End, they fit a bit boxy. Comes in both a traditional and a tailored fit.
Vermont Flannel Company - Probably the best bang for your buck. The fit is very big and boxy, though they do produce a slim fit that hasn't made it to their website yet. Best option would be to phone in and make an order, or visit them in person in possible.
Middle
Pendleton - The classic, iconic wool flannel. Very warm and reasonably thick. You can get them cheaper by sifting through Ebay and vintage stores. Comes in both a Fitted and Regular cut.
Woolrich - The other classic, iconic wool flannel. A bit more rough and scratchy than Pendleton, but still very warm. They also make cotton flannels at a lower price point if you prefer. Fit is boxy.
Filson - A nice, comfortable cotton flannel with a traditional fit.
Five Brother - A little hard to track down, but worth it. Very heavy, well-fitting flannels from a workwear brand.
John Lofgren - Thick cotton flannels with short, vintage cuts, again from a more traditional workwear brand.
High
Flat Head - A japanese label that has an infatuation with american workwear and biker culture. Along with the next brand on this list, these are considered some of the best possible flannels that money can buy. Comes in both a slim fit and a slightly more regular fit ("Glory Park").
Iron Heart - Another japanese workwear brand. They are mostly known for their insanely tough denim, but they also produce some insanely tough flannels as well. Along with Flat Head, they are considered some of the best flannels out there.
Engineered Garments - Fantastic flannels. Not quite as thick or tough as Iron heart or Flat head, but made with just as much care and attention. These shirts have tons of small, interesting details. The fit is slightly traditional and boxy. Made in NYC, designed by Daiki Suzuki who was formally the designer for Woolrich Woolen Mills.
Nigel Cabourn - Nice, thick flannels with lots of interesting details. Shirts are often reversible. Made in Japan.
Neighborhood - Nice flannels from japan. Very popular among certain streetwear crowds. Shirts will often have interesting details, such as prints.
Visvim - Great flannels from another japanese brand. Made from luxurious cotton, touted as "the highest grade cotton in the world." Designed by Hiroki Nakamura, who has an infatuation with 60's Americana, workwear and hippie subcultures as well as Native American motifs.
Needles - Awesome flannels, patched together from recycled vintage and surplus flannels. Each shirt is unique.
Lad Musician - Interesting flannels from designer Yuichi Kuroda. Lad Musician's mission statement is "clothing fit for musicians." Designs take heavy inspiration from various musicians as well as punk and rock subcultures. The fits are very slim and tailored, but cut for mobility. The flannels themselves are often made from a unique blend of rayon and cotton, and can often be found with details such as nonexistent collars- the shirts can sometimes also be bought in varying lengths.
Junya Watanabe - Interesting flannels from Junya Watanabe, a designer from Comme Des Garcons. These flannels are usually made from wool, and usually involve some sort of patchwork design to them.
Saint Laurent Paris - Slim, tailored flannels from designer Hedi Slimane. Like Lad Musician, the brand takes heavy inspiration from rock and punk subcultures and the flannels are made from a rayon/cotton blend.
If you have any other recommendations or extra information about any of these brands please leave them in the comments.