I have not done all of the level sets yet, but I have got some thoughts about some of them:
(1995) Jan Hubicka's level 016(Mine after earthquake) is very hard. I had no idea how to get out of the section with a magic wall. In fact, I had to load the solution tape several times to get it! And I thought that every level is easy given enough time with the recorder!
About the level set itself, I like it. It has been pretty fun and creative. Funnily enough, I did not guess what does "Sipkova Ruzenka" mean, even though Czech and Polish are both West Slavic languages!(I am Polish)

(1998) I like Ben Braithwaite's level set a lot. It has been very fun, and I like the "it does not have to be challenging to be fun" approach. Some levels can be exited at the start, but you can explore(examples: 000(Come past it!), 005(Collect lots of points), 023(Fun spiral), 053(Don't hurry), 093(Kill the crunchers)). Many levels stood out to me: 022(Doom), 030(Long way up), 061(Doom two), 062(Doom doom doom doom doom doom), 070(Tutti frutti), 078-092(#-#-M) and many more!

(2000) I love Arno Luppold's level set! It simply focuses on fun, like Ben Braithwaite's level set! I also love the drawings! My favourite levels: 020(Emerald Mine II), 041(A castle) and 047(Happy new year!).

(2001) Helge Hafting's Avalanche is quite big, crazy and fun.
(2006) Sascha Mücke has done a great job using custom elements. My favourite level of the set is the last one(008)(CE-experiment).
Another noteworthy thing is that some contributors have put in their contact info(e-mail or phone number) and I thought: "Oh boy, if RnD becomes popular, then they are going to have a bad time."

So, what do you think?