In your opinion which are the best agencies speaking of submission process?
Which one has the easiest and less time consuming process?

First bit of advice... easiest and fastest doesn't matter. 

It's all about ROI... Return On Investment.  I'd say Graphic Leftovers is easiest... simply use their web-based uploading window, upload a bunch at once, and no follow-up needed.  I think the team there is great, but most days, I have no sales.  They're a site to watch, though, so I keep uploading despite the low ROI at the moment.

Here's a better way to answer the question that might help you... who is fastest/easiest AND who delivers ROI.

I'll rate the sites that matter on a scale of 1 to 10... 10 being fast and easy, and high revenue... 1 being painful upload process, and pitiful sales...

Shutterstock:  Uploading - 5, ROI - 10

Fotolia: Uploading - 5, ROI - 9

iStock: Uploading - 1, ROI - 5

Dreamstime: Uploading - 5, ROI - 7

Bigstock : (I'm in the Bridge to Bigstock program, so I can't recall what they're like, but I'd say ROI is 4.)

123RF: Uploading - 7, ROI - 6

CanStock: Uploading - 6, ROI - 5

Veer: Uploading - 2, ROI - 2

Thanks!
Yep, I'm already subscribed to the Top Tier and some others, I just want to know if there are other smalle agencies with a submission process really fast and simple (so you don't mind if sales are slow).

Graphic Leftovers seems interesting, I'll give it a try!