Curious to know what favorite Rod, wand, Staff, or other weapons with crazy specials effect or Stat you have morphed your Pact weapon into.   

Rod of the Pact Keeper comes to mind as very awesome

Shatterspike is interesting.  its +1, you can auto Crit against "object" and choose to deal slashing or Bludgeoning  (and only uncommon too)

Skyblinder staff:  +1 bonus to attack and damage rolls made with this magic weapon. If it hits an object, the hit is automatically a critical hit, and it can deal bludgeoning or slashing damage to the object (your choice). Further, damage from nonmagical sources can't harm the weapon.

Staff of Defense:   While holding the staff, you have a +1 bonus to your Armor Class.  The staff has 10 charges, which are used to fuel the spells within it. With the staff in hand, you can use your action to cast one of the following spells from the staff if the spell is on your class's spell list: mage armor (1 charge) or shield (2 charges). No components are required.The staff regains 1d6 + 4 expended charges each day at dawn. If you expend the staff's last charge, roll a d20. On a 1, the staff shatters and is destroyed. Versatile.  This weapon can be used with one or two hands. A damage value in parentheses appears with the property—the damage when the weapon is used with two hands to make a melee attack.

Tentacle Rod :   While holding the rod, you can use an action to direct each tentacle to attack a creature you can see within 15 feet of you. Each tentacle makes a melee attack roll with a +9 bonus. On a hit, the tentacle deals 1d6 bludgeoning damage. If you hit a target with all three tentacles, it must make a DC 15 Constitution saving throw. On a failure, the creature's speed is halved, it has disadvantage on Dexterity saving throws, and it can't use reactions for 1 minute. Moreover, on each of its turns, it can take either an action or a bonus action, but not both. At the end of each of its turns, it can repeat the saving throw, ending the effect on itself on a success.  (not sure of you morphed this into a Greatsword if this damage would become 2d6...?)

I have a Dragonborn Hexblade with Tiamat as the patron. His Hexblade weapon is a 5-headed flail, each flail head is a carved gemstone made to look like the 5 main chromatic dragon heads (Ruby for red, Saphire for blue, etc). It started out as his Rod Arcane Focus and the Dragonborn Hexblade went around “Thanos Style” to find the other gems flail heads to add the weapon. Like Infinity Gauntlet meets the Flail of Ages.  It’s not powerful (...yet) but each flail head adds 1 elemental damage (of its respective dragon type) and it has the “unbreakable, guardian and muttering minor properties” from the DMG. The flail heads animate and talk to the dragonborn warlock. They will even warn him of incoming danger and enjoy teasing and cracking jokes at the parties expense. 

The flail “absorbs” magical items by animating and eating them like piranha’s skeletonizing a cow. I did that to a scimitar of speed now it is a +2 flail of speed that does 5 elemental damage.