Hi i find best by pagerank or by date submitted as if you do it alphabetically you will find lots of people will put “best” before there title or “A1” like “A1 online casinos” and stuff like that to make sure there in a top position when thats not even there names – i have 1 directory where this happened alot as i have near 10,000 submissions so i changed from alphabetically to pagerank 
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but the other 2 have drawbacks also as when your directory is quite full, if its by pagerank or date submitted new website owners wont want to pay a fee if they know they are going to be burried deep within subpages – but i still prefer these two ways – date submitted is prob the most fair.