I was curious about this so I ran a program to work out things like power + toughness per mana by set. There are some results below. It's completely unscientific and things like Death's Shadow count as 13 while Omnath, Locus of Mana counts as 1 so take it with a pinch of salt.
This is from a list of 84 "sets" for which I have data including From the Vault and Duel Decks. I found it interesting that so few sets (but some!) had higher average power than toughness (second-last list). There's 8 in total.
Worldwake has the highest power-per-mana of any set ever produced.
I have the data and the program should anyone be nerdy enough to want to know something else.
Highest average power + toughness per mana:
Arabian Nights 1.21
Worldwake 1.15
Weatherlight 1.11
Coldsnap 0.87
Duel Decks: Garruk vs. Liliana 0.85
Beatdown 0.85
Urza's Saga 0.83
Conflux 0.81
Fallen Empires 0.80
Revised Edition 0.78
Masters Edition 0.78
Highest average power + toughness:
From the Vault: Dragons 10.62
Beatdown 7.39
Alara Reborn 6.12
Masters Edition III 6.09
Coldsnap 5.77
Conflux 5.74
Chronicles 5.69
Legends 5.69
Portal 5.58
Ravnica: City of Guilds 5.58
Duel Decks: Garruk vs. Liliana 5.56
Lowest average power + toughness per mana:
Duel Decks: Divine vs. Demonic 0.29
From the Vault: Dragons 0.31
Masters Edition III 0.32
Planechase 0.32
Legends 0.34
Scourge 0.39
Portal Three Kingdoms 0.39
Torment 0.40
Unglued 0.43
Mirrodin 0.43
Lowest average power + toughness:
Duel Decks: Elves vs. Goblins 3.39
Battle Royale 3.55
From the Vault: Exiled 3.67
Fallen Empires 3.70
The Dark 3.73
Stronghold 3.75
Exodus 3.76
Premium Deck Series: Slivers 3.76
Odyssey 3.96
Tempest 4.15
Urza's Legacy 4.26
Highest power-per-mana:
Worldwake 0.57
Weatherlight 0.49
Arabian Nights 0.48
Beatdown 0.46
Unglued 0.43
Masters Edition 0.4
Urza's Saga 0.39
Coldsnap 0.39
Duel Decks: Garruk vs. Liliana 0.37
Shadowmoor 0.37
Stronghold 0.36
Highest toughness-per-mana:
Arabian Nights 0.72
Weatherlight 0.62
Worldwake 0.58
Revised Edition 0.50
Alliances 0.48
Duel Decks: Garruk vs. Liliana 0.48
Fallen Empires 0.48
Coldsnap 0.48
Limited Edition Alpha 0.47
Limited Edition Beta 0.47
Unlimited Edition 0.47
Conflux 0.45
Highest average creature power:
From the Vault: Dragons 5.31
Beatdown 3.83
Alara Reborn 2.99
Masters Edition III 2.87
Conflux 2.81
Portal 2.79
Coldsnap 2.78
Duel Decks: Garruk vs. Liliana 2.74
Duel Decks: Divine vs. Demonic 2.68
Chronicles 2.66
Legends 2.64
Shadowmoor 2.64
Shards of Alara 2.63
Ravnica: City of Guilds 2.61
Scourge 2.61
Worldwake 2.61
Prophecy 2.61
Fifth Dawn 2.61
Morningtide 2.58
Starter 1999 2.57
Weatherlight 2.51
Highest average creature toughness:
From the Vault: Dragons 5.31
Beatdown 3.57
Masters Edition III 3.22
Alara Reborn 3.13
Legends 3.05
Chronicles 3.03
Coldsnap 2.99
Ravnica: City of Guilds 2.97
Conflux 2.93
Duel Decks: Divine vs. Demonic 2.84
Duel Decks: Garruk vs. Liliana 2.81
Fifth Dawn 2.81
Prophecy 2.8
Portal 2.79
Revised Edition 2.77
Duel Decks: Jace vs. Chandra 2.76
Limited Edition Alpha 2.76
Limited Edition Beta 2.76
Unlimited Edition 2.76
Lowest average creature power:
Fallen Empires 1.65
Duel Decks: Elves vs. Goblins 1.7
Battle Royale 1.73
Exodus 1.76
Lowest average creature toughness:
Duel Decks: Elves vs. Goblins 1.68
Battle Royale 1.82
Premium Deck Series: Slivers 1.88
The Dark 1.97
Sets with average creature power and toughness both below 2:
Duel Decks: Elves vs. Goblins
Battle Royale
The Dark
Stronghold
Sets where average power is greater than average toughness:
Unglued
Beatdown
Saviors of Kamigawa
Planechase
Urza's Legacy
Visions
Morningtide
Duel Decks: Elves vs. Goblins
Sets where average toughness is more than 0.5 higher than average power:
Arabian Nights 0.6
Fourth Edition 0.62
Limited Edition Alpha 0.67
Limited Edition Beta 0.67
Unlimited Edition 0.67
Revised Edition 0.7
Alliances 0.71
Antiquities 0.82