Miniseries
Night 1
Night 2
Season 1
1.01 33
1.02 Water
1.03 Bastille Day
1.04 Act of Contrition
1.05 You Can't Go Home Again
1.06 Litmus
1.07 Six Degrees of Separation
1.08 Flesh and Bone
1.09 Tigh Me Up, Tigh Me Down
1.10 The Hand of God
1.11 Colonial Day
1.12 Kobol's Last Gleaming, Part I
1.13 Kobol's Last Gleaming, Part II
Season 2
2.01 Scattered
2.02 Valley of Darkness
2.03 Fragged
2.04 Resistance
2.05 The Farm
2.06 Home, Part I
2.07 Home, Part II
2.08 Final Cut
2.09 Flight of the Phoenix
2.10 Pegasus (56 minute extended version)
2.11 Resurrection Ship, Part I
2.12 Resurrection Ship, Part II
2.13 Epiphanies
2.14 Black Market
2.15 Scar
2.16 Sacrifice
2.17 The Captain's Hand
SPEC Razor (TV movie - 99 minute extended version. Note: This was originally broadcast just before Season 4, but chronologically it fits here, telling more of the Pegasus's story. Some people argue it's better to watch after Season 3 ends, as originally broadcast.)
Optional: Razor Flashbacks (This was billed as a "7 episode web series", but really they were just deleted scenes from the broadcast version of Razor. In fact, most of these scenes are now reintegrated into the extended version of Razor, making what's left even more unessential. They are mentioned here only for the sake of completeness, and to reduce confusion; they are far from necessary. The only "episodes" not reintegrated are 1, 2 and some parts of 7. Note: These "episodes" were originally released online before Razor was broadcast, but the extended version means it makes a more sense to view them afterwards.)
2.18 Downloaded
2.19 Lay Down Your Burdens, Part I
2.20 Lay Down Your Burdens, Part II
The Resistance
A 10 episode web-based series bridging seasons 2 and 3. (25 mins.)
Season 3
3.01 Occupation
3.02 Precipice
3.03 Exodus, Part I
3.04 Exodus, Part II
3.05 Collaborators
3.06 Torn
3.07 A Measure of Salvation
3.08 Hero
3.09 Unfinished Business (70 minute extended version - Note: Not included on Region 2 DVDs, but is included on ALL Bluray releases.)
3.10 The Passage
3.11 The Eye of Jupiter
3.12 Rapture
3.13 Taking a Break From All Your Worries
3.14 The Woman King
3.15 A Day in the Life
3.16 Dirty Hands
3.17 Maelstrom
3.18 The Son Also Rises
3.19 Crossroads, Part I
3.20 Crossroads, Part II
A lot of fans would argue that it's worth watching the end of Razor again at this point (specifically the scenes featuring the Hybrid), as this was where Razor was originally broadcast, and its ending sets the tone for Season 4.
Season 4
4.01 He That Believeth In Me
4.02 Six of One
4.03 The Ties That Bind
4.04 Escape Velocity
4.05 The Road Less Traveled
4.06 Faith
4.07 Guess What's Coming to Dinner?
4.08 Sine Qua Non
4.09 The Hub
4.10 Revelations
4.11 Sometimes a Great Notion (Actually the first episode of "Season 4.5")
The Face of the Enemy
A 10 episode web-based series (although it's really just a single mini-episode). (36 mins.) These episodes have not been included on any DVD or Bluray releases, but if you live in the US you can view from for free at Hulu.com. They are not presently available anywhere else in the world. (I highly recommended you do your best to find them.)
Season 4 Continued (aka "Season 4.5" or "The Final Season")
4.12 A Disquiet Follows My Soul (53 minute extended version - only on Bluray releases)
4.13 The Oath
4.14 Blood on the Scales
4.15 No Exit
It has been proposed that you might watch The Plan (see below) at this point, and learn more about the Cylons' story from their point of view. It might be preferable to do it now, before the series has finished, rather than later once everything is all tied up. It all depends if you don't mind going back to the beginning before the end...
4.16 Deadlock
4.17 Someone to Watch Over Me
4.18 Islanded In a Stream of Stars (62 minute extended version - only on BluRay releases)
4.19 Daybreak (150 minute extended version - only on BluRay releases)
The Plan
A DVD/Bluray movie that shows (approximately) the first two seasons from the Cylons' perspective. (It might be possible (and preferable) to watch this after No Exit, but I'm unsure exactly when would be best. If anyone has any recommendations, I'll consider moving it.)
Caprica
An entire TV show(!) set 58 years before the events of Battlestar Galactica, and revealing the events surrounding the creation of the Cylons. (Although it's worth noting that you don't have to have seen BSG to watch Caprica.)