Here is a list of the top 5 non-legendary Pokémon, excluding more powerful alternate forms such as megas (regional forms count), with the highest in each base defensive stat:
Note: this answer assumes Sub-Legendary and Mythical Pokémon count as legendary, but nothing else does.
Defense:
- 1st: Stackataka at 211
- 2nd: Steelix at 200
- 3rd: Tied between Hisuain Avalugg and regular Avalugg at 184
- 4th: Tied between Aggron and Cloyster at 180
- 5th: Bastiodon at 168
Special Defense:
- 1st: Florges at 154
- 2nd: Tied between Goodra, Hisuian Goodra, Carbink, and Probopass at 150
- 3rd: Toxipex at 142
- 4th: Mantine at 140
- 5th: Tied between Aegislash (Shield Form) and Mantine at 140
Source: Pokedex With Stats-- Pokemon Database
Hope I helped!