comfortable dining chair  

There are a lot of them...
Personally, I like the Eames Wire Chair, Plastic side Chair and Saarinen Tulip chairs. I think they're shape is so perfect you can never sit uncomfortable on them. I am very fond of the design of the Vitra 03 chairs, but never sat on one long enough to tell if they are really comfortable... If you find out, let me know though,... I think they would match perfectly with my table for tools.

Dining chairs
Brno, I love the springy little bounce

the BEST
No contest - The two most comfortable dining chairs on the planet are the iconic (albeit over-copied) Series 7 by Arne Jacobsen, and the Sim by Jasper Morrison.

Four Lay-Z-Boys around a...
Four Lay-Z-Boys around a table.

What wrong with you? Cant't...
What wrong with you? Cant't you think for yourself? Are you missing an ass and can't sit in a chair?
What you do is go to a shop, sit in some chairs and then.... make your own decision.

Seriously
I feel a couple of suggestions here have been made based on aesthetic bias. My opinion based soley on comfort would be:
1. Saarinen Executive arm and side chairs.
2. Saarinen Tulip series. The plastic used with these are more pliable than an Eames armshell and they have seat pads.
3. Padded Eames armshell or sideshell. Many people prefer shells sans pad, but if you find a cool, clean matching set it looks great and feels nice on the tush.
4. Eames aluminum group arm or side chairs.
5. Eames soft pad arm or side chairs.
Obviously arms add more support.
I use Bertoia side chairs with seat pads. I love them. Obviously a full pad would be more comfortable.

Wishbone
I think that the CH24 chair or "Wishbone Chair" by Wegner is very comfortable for dinning. Plus I think they look sexy.
I know andyallen agrees

sexy?
it cracks me up when i hear someone say that furniture looks sexy. perhaps we should start another thread for this, but is there a sexy aspect to furniture ... as in an actual ability to turn one on? i have heard people say that many times, but i just don't know if i can relate to that description. i seem to think in terms or 'good' and 'bad' design.

p.s.
I am on this "sexy" kick. I have lately been describing everything "good" as "sexy." I used to say that things were "rad" or "Awesome"
Furniture rarely gives gets me aroused.

Clear Acrylic Telescoping Chair
I don't know that they're the most comfortable, but for space saving, they fold and store away. For our dining area we have a set of clear acrylic telescoping chairs (see link). Some might even call them "sexy".

wishbone chairs - slutty
I think they are sexy, just a little too easy, anyone who wants to can enjoy them. But yea, I don't think we literally mean sexy, for example, lately I have been using banging when I describe something, for example: her backyard's BANGIN' like a benzy

I consider Brno chair by...
I consider Brno chair by Mies to be the best choose. Probably I find it confortable also because it is so fine....