When casting on yourself, you're often trying to reduce your vulnerabilities more than gain advantages. To that end, becoming an ooze is a great option as there are generally very few interactions with the ooze type. I might aim for a construct or undead for a single fight, but if I am going to mask myself permanently, I'm oozing.
If you want to trick someone into accepting the spell and then using it to mess with them, undead may be the way to go. If you use the 2014 Oathbreaker paladin and have one in your party ... You can essentially take control of any creature with a CR less than your level. In theory, once they agree, you can keep them under control forever (subject to DM interpretation).
In terms of items, 75% of cursed items in my game have their aura masked and I determine that this has been placed on 10% of uncommon items, 25% of rare, 40% of very rare and 50% of legendary items. Magic items tend to be very old and this type of protection makes a lot of sense for the enchanting wizard to put into place. [I drop hints that an item is magical and then it doesn't come up with a magical aura ... which results in PCs sometimes overlooking them and leaving them behind, but other times the group decides they need to cast dispel magic on every treasure hoard).
