Cliffs -
#1 Doesn't really follow the rule of thirds, seems a bit too centred.
#2 Best of the cliff images imo, sharp, vibrant colours and better composition.
#3 Seems a little blurred.
#4/5 Both pretty good. Can't really see how to improve them much.
Dragonfly -
#1 Is out of focus, shame because it would have made a great picture.
#2 Is much sharper/in focus but isn't at an ideal angle. Still quite nice though.
Random caves -
#1 There doesn't seem to be any contrast so the stalactites don't really stand out.
#2 Better, you haven't used a flash in this and the shadows make them much more defined.
#3 Little bit blurred due to the long exposure.
Kefalonia -
#1 Little blown out at the top, otherwise good.
#2 Blurry and overexposed I'm afraid. To avoid this try using a higher ISO and a lower F/stop (say 400ISO, 3.6F/stop if possible). This will let you use a quicker shutter speed and should stop the problem with blurring
#3 Quite nice, a little over exposed but nothing major. This is probably fixable in PS. Here is a very quick attempt:
#4/5 Again not bad but a little blown out at the top.
Last picture - Really like this, easily the best of them all imo
Not bad at all for a first attempt!