Assuming that you are only interested in monsters from official WotC materials, D&D Beyond lists only two monsters at CR 30:
1. Tarrasque
2. Tiamat
But what will be the most challenging to your party depends on, well, your party. Lower CR creatures can prove a greater challenge based on spell ability, lair actions, etc.
The Tarrasque, for example, may not be that deadly for a high powered party that can fly and make ranged attacks with magic bows and arrows. It'll just be a slow slog to chip away at that big bag of hit points.