I'd go:
1. France
2. Spain
3. England (PL)
4. England (Championship)
France 1st because the steals from that country often go on to be stars in the PL and the price ceiling means the gambles are worth it. The settling in period from Ligue Un to the PL is relatively short.
Spain, more about whether the player settles to the climate and life here than having any worries about them on the pitch. When players come from Spain and have appetite for the English game they tend to thrive.
England. For the prices paid, it's not that good a league to buy from. Some players bought for top whack go forth and prosper and sometimes, there are great bargains, but everything comes at a premium and if a player doesn't work out after a bigger move, the buying club have to take the financial hit as nobody will match the wage and they've got to sell on at a reduced rate.
England Championship. I think this will be the market with the most room for growth in the upcoming years as more PL clubs look to plunder from it. At the very least, you're getting grafters who will fight and make for difficult opponents, at the other end, you could snatch up talent that goes on to be worth triple or quadruple what was paid, and further to that, the likelihood of them wanting to move 'abroad' is slimmer than any of the others. Would be higher up in my list if the sample size was as extensive as the others, but it isn't.
I think all the other leagues sit pretty comfortably side-by-side in the sense that a gem may come along or you could get a dud. Your old sure things like the Eredivisie are no longer that because the league is a shadow of what it was 20, 15, or even 10 years ago; Italians are notorious for not settling, or plain not leaving Serie A; South Americans rarely move directly to the PL; Portuguese and Belgians are not bought directly in enough numbers to have PL clubs scouting there relentlessly; Scotland is a little bit like the Championship, but still not a market PL clubs eagerly keep a watchful eye over and the Eastern European leagues are a much of a muchness. The rest of the world has the odd buy, but you've got to have one hell of a scouting network and confidence in a talent settling to take them from vastly different cultures and lifestyles to England.
edit: forgot Die Bundesliga... seems like a mixed bag to me. Talent transitioning from there to here is not guaranteed, but this one is a market with growth potential, too. I wouldn't be as doubtful of a Bundesliga player coming good here as I would some of the leagues, and I'd certainly buy from there before Serie A or Eredivise at the moment.
