I'd like to see some numbers to back this up. Not saying I disagree, just curious to see the math.
Okay.
By the way, both you and Sessadore did your calculations with level 1 characters, where of course the effect of weapon damage dice will be the greatest and you will have the smallest amount of static damage bonuses. Let's do some level 30 calcs.
Let's assume that a baseline of +2 proficiency with +8 attack stat modifier will give 50% hit.
Tempest Fighter: Reckless Double Sword +6, Weapon Focus, Agile Tempest, Pit Fighter (assume +4 Wis), Iron Armbands of Power, Marked Scourge, Heavy Blade Mastery
40% miss, 50% hit, 10% crit
(0.4 x 0 + 0.5 x 48 + 0.1 x 82) x 2 = 64.4 DPR with Double Strike
Great Weapon Fighter: Bloodclaw Fullblade +6, Weapon Focus, Pit Fighter (assume +4 Wis), Iron Armbands of Power, Marked Scourge, Heavy Blade Mastery. No Power Attack, 'cause it's a DPR loss at this high level of damage-on-hit.
40% miss, 50% hit, 10% crit
0.4 x 8 + 0.5 x 62 + 0.1 x 110.5 = 46.85 DPR with Reaping Strike
Stormwarden Ranger, current king of DPR: Reckless Scimitars +6, Weapon Focus, Lethal Hunter, Scimitar Dance, Storm Warden (assume +8 Dex), Iron Armbands of Power, Heavy Blade Mastery.
50% miss, 40% hit, 10% crit
(0.5 x 8 + 0.4 x 36 + 0.1 x 70) x 2 + 16 + (0.75 x 13.5) = 76.925 with Twin Strike
EDIT:
Wait a minute, Great Weapon Fighters have a nifty new at-will too, Brash Strike. Let's see what it does. Change weapon to Mordenkrad, add Hammer Rhythm, add in the +2 hit and +Con damage, all for the cost of granting Combat Advantage to enemies (no big deal usually for a defender, and might actually be good for "aggro" as per Cadfan's post above)...
35% miss, 55% hit, 10% crit
0.35 x 8 + 0.55 x 73 + 0.1 x 102 = 53.15 DPR with Brash Strike. Still not as good as Tempest, but getting much better.