It's useful to know the total market size of regions, so here are the 23 regions of empire space sorted both by sovereignty (Caldari, Minmatar, etc) and by market order size for all 23 regions. The sovereignty list is further broken down into which pirates are in the area (helpful for deciding what to buy there). All data (several days old) was pulled from
http://eve-marketdata.com/.
Sorted by sovereignty
Caldari Space (Guristas)
The Forge 130,443 orders 30,137,310,597,270 ISK
Lonetrek 40,992 orders 2,835,952,919,398 ISK
The Citadel 29,557 orders 2,108,965,930,169 ISK
Black Rise 14,669 orders 923,661,026,698 ISK
Minmatar Space (Angels)
Heimatar 47,912 orders 4,249,919,677,100 ISK
Metropolis 43,157 orders 3,412,777,388,159 ISK
Molden Heath 10,543 orders 589,852,832,717 ISK
Gallente Space (Serpentis)
Sinq Laison 54,149 orders 4,842,811,416,077 ISK
Essence 33,756 orders 2,155,119,081,662 ISK
Placid 23,634 orders 1,388,225,291,147 ISK
Everyshore 15,663 orders 899,629,938,222 ISK
Verge Vendor 20,012 orders 881,916,332,290 ISK
Solitude 6,107 orders 284,916,819,863 ISK
Amarr Space (Sanshas)
Domain 67,709 orders 7,884,414,092,573 ISK
Tash-Murkon 21,251 orders 1,424,221,181,822 ISK
Derelik 18,960 orders 1,252,071,888,805 ISK
Devoid 9,516 orders 664,747,633,607 ISK
Amarr Space (Blood Raiders)
Khanid 13,724 orders 1,101,590,959,464 ISK
Genesis 14,121 orders 700,858,157,625 ISK
Kor-Azor 11,339 orders 668,413,268,198 ISK
Kador 13,197 orders 680,114,499,773 ISK
The Bleak Lands 6,478 orders 368,640,361,594 ISK
Aridia 4,410 orders 325,079,022,907 ISK
Sorted by total sell order value
The Forge 130,443 orders 30,137,310,597,270 ISK
Domain 67,709 orders 7,884,414,092,573 ISK
Sinq Laison 54,149 orders 4,842,811,416,077 ISK
Heimatar 47,912 orders 4,249,919,677,100 ISK
Metropolis 43,157 orders 3,412,777,388,159 ISK
Lonetrek 40,992 orders 2,835,952,919,398 ISK
Essence 33,756 orders 2,155,119,081,662 ISK
The Citadel 29,557 orders 2,108,965,930,169 ISK
Tash-Murkon 21,251 orders 1,424,221,181,822 ISK
Placid 23,634 orders 1,388,225,291,147 ISK
Derelik 18,960 orders 1,252,071,888,805 ISK
Khanid 13,724 orders 1,101,590,959,464 ISK
Black Rise 14,669 orders 923,661,026,698 ISK
Everyshore 15,663 orders 899,629,938,222 ISK
Verge Vendor 20,012 orders 881,916,332,290 ISK
Genesis 14,121 orders 700,858,157,625 ISK
Kador 13,197 orders 680,114,499,773 ISK
Kor-Azor 11,339 orders 668,413,268,198 ISK
Devoid 9,516 orders 664,747,633,607 ISK
Molden Heath 10,543 orders 589,852,832,717 ISK
The Bleak Lands 6,478 orders 368,640,361,594 ISK
Aridia 4,410 orders 325,079,022,907 ISK
Solitude 6,107 orders 284,916,819,863 ISK
The bottom four + Placid and Black Rise are primarily PvP markets. This doesn't necessarily mean PvP is bad for business, it just means business is riskier in PvP areas.
I've always said Lonetrek is one of the best trading regions in the game. This list backs me up on that. Not only is Lonetrek tops among regions without a main hub but if you subtract the numbers for the main hubs themselves from the numbers for their regions, Lonetrek is ahead of the main hub regions too, even beating out The Forge (Jita region) by a few hundred million.