It's not a CustLemm/LemEdit pack, but I was a huge fan of a pack for the now defunct Cheapo Copycat game called "Why Bother?" by "The Purpletrator" [I am not entirely sure I am spelling that right]. Absolutely beautiful levels that were very challenging and satisfying to solve. Only qualm with it is the creator's fondness for hidden traps, but that's a minor thing.
Martin Zurlinden's pack was the pack that got me into level design in the first place. A lot of tough levels there, also. Never tried DoveLems, I'll have to have a look at it.
I can't really name one least favorite pack, just because there's several on the Levelpack Database that are just hideous and are pretty obvious first attempts at level design. Flag/object spam (as CS mentioned), jarringly badly placed terrain, trivial solutions... ugh. Just, no. I don't remember what any of them are called but I'm pretty sure no one who posts here made any of these wrecks.