Zemmings Vol. 1 is also a strong recommendation. It only has eight levels, but I'd say the levels are not overly hard and a lot of fun to solve. This one will also give you a taste of one of the
best graphic sets designed by the creator of the pack himself!
Revenge of the Lemmings will not work on newer versions of NeoLemmix (last update for it was January 16, 2016 which was built on an older version of the Flexi Player), especially how some of the levels rely on the Cheapo gimmick which has been removed.
If you want to play
the Lemmings Plus series as well (also highly recommended!), Lemmings Plus I is also a great pack for new users, it has a much fairer difficulty curve and the levels only get much harder in the last two ranks. The same applies to Lemmings Plus Omega for those who want to accustom themselves to the new NeoLemmix features such as objects and skills.