Man, it really is up to you because everyone has different opinions about what Easter Egg is the hardest. But i can give a small description helping you make your choice.

ZiS: very few steps but pretty hard and RNG. Basically a game of defend which gets harder. Bossfight involves simon says training and spraying

Shaolin: A lot of small steps which aren't really hard but forces you to do the easter egg in a higher round than usual. Bossfight is quite easy, involves some aiming skills but nothing too hard and crafting the special weapon of this map

Attack: average set of steps but some steps are tedious and make you travel the map multiple times and involves a bit of math but nothing too complicated. Bossfight involves escorting, a bit of aiming skills and a parkour

Beast: The most simple easter egg steps, collecting 4 disks which need to be inserted in order. Bossfight is hard for the unexperienced player, you get quickly overrun. But bring high fire power and know how to train in that boss area and eventually it will be an easy bossfight