Most of the enemies in buried cathedral are inmune or high resistant to physical damage so you have to pick a sword with elemental damage obviously, the thing is that the weakness and resistances of these enemies are very different from one to another so it might be a problem pick only one sword. You should carry different swords and equip/unequip with hotkeys.
At level 200: Summerblade for Ripper Spectre and Burster Spectre (120%) and Winterblade for Gazer Spectre (130%). For Arachnophobica it doesn't matter because is neutral to both. Carrying only Summerblade you will have a hard time against Gazer Spectre (30%) and with only Winterblade you will have with Burster Spectre (30%)
At level 250 you could use gnome sword, energy damage is strong on Ripper Spectre (110%) and neutral on the others spectres but Aracnophobica is resistant to it (50%). In my opinion not the best choice but if you for some reason only want to carry one sword this one maybe would do the trick.
At level 270 you could use lion long sword, similar to gnome sword but this one has earth damage all enemies are neutral to earth damage except Ripper Spectre (80%) best option for carring only one sword but I will Stick with Summerblade and Winterblade though.
At level 300 you have naga sword with 44 ice damage and +3 sword fighting is better option than Winterblade because it has only 40 ice damage and +1 sword FIghting.
At level 400 you have Soulcutter the death damage has the same resistance and weaknesses that energy damage so not really a good option.
Finally, at level 600 you have Sanguine or Grand Sanguine Blade best sword in game and Fire Damage bassed.
So, I listed all options but the wisest choice is pick the best fire damage based sword and the best ice damage based sword available and equip/unequip based on the weaknesses of your enemies.