I've already looked into jonatan's awesome maps and will be keeping an eye out for his new stuff, but I would also like to download maps of that level (of difficulty) made by other users as well..

Thanks in advance.

Yeah Nys-Series are AWESOME... you'll have a loooooot of fun if you like that kind of challenging maps. Also, if you already know jonatan's maps, my remaining recommendations (at a first shoot only... I'll post more when I remember them...) would be:

1. Engineer (by Ruien)
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=701

2. Chander's maps:
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=1251
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=1222
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=1261
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=1331
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=1360

3. Omnicoder's maps:
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=617
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=508
   (of course, check out for Portal 1 maps as well)

4. Damagepy's maps
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=659
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=330
   (of course, check out for Portal 1 maps as well)

5. InvisiblePanda's maps:
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=1364
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=1090
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=705

6. Vengeance (by satchmo):
   http://forums.thinking.withportals.com/community-releases/vengeance-t3934.html

7. Blue Walls (by RubyCarbuncle):
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=694

8. Portal Raider Toxic (by Haggis):
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=1248

9. Bridges to a new tomorrow (by Zhan):
   http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=706

10. Aperture Experimental Labs (by doubledispatch):
    http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=1249

11. Delta Bunsen! (by Lobster)
    http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=729

Some of above are just tricky, not necessarily hard, but you will sure like them anyway. Others are REALLY difficult maps.

Wait your comments!

EDIT: all of them are SINGLE player maps... I've played many coop with a good level of difficulty... Let me think of it and will post them shortly.

(Mi amigo Carlos me va a dar en la jeta!)

PSICODELIA by Carlymx:

http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=1000