This is hugely subjective. The 'best' for one person may be anathema to another. Personally, I would avoid micro-stock sites, that have vast collections of images from amateur photographers selling at a couple of quid each. You'd have to submit hundreds of images just to appear in searches, let alone make any sales.

I use Alamy for my photos, though I don't make regular sales. When I have sold a photo, it's usually been for about £100 or more. They will take any photo that is technically excellent (i.e. they don't filter by percieved artistic merit) and you specify when you upload the image whether you want to sell royalty-free or licenced.

A lot of newspapers get their images from Alamy - one of mine ended up in the Guardian. They do appear to have a significant customer base, however most of the time you won't know who has purchased an image. The only reason I knew of the one in the Guardian was because I came across it by accident - that was a nice surprise!

Anyway, they do pay well - they take 40% commission (though that increases to 50% for 'Novel Use', and 60% for distributor sales), meaning you get more than half of the sale price for the majority of sales, and they allow you to see views, zooms and purchases of all your images on a day by day basis, including the keywords that were used when finding your photos, which helps in getting your keywords right. You can change photo details at any point after uploading (except the licence type), and you can indicate whether you have model and/or property release on photos that require it.

Check the BAPLA website for a comprehensive list of photo libraries in the UK.