what level is the best when chalenging the battel tower in crystal

Strangely,for me the hardest level was 10,because the enemies had strong movesets (Blizzard,Flamethrower,Thunder,Solar Beam...)and in this level Pokémon easily went ko.It's less difficult the level 100,in comparison with.
The best is the 50 or 40,if your Pokémon are medium-strong.