Pinging your opponent and dealing direct damage doesnt count
In terms of a serious threat in blue that costs five mana Meluko trumps all the alternatives however when facing the likes of Banelsayer Angel, Wolfir Silverheart, Seige-Gang and Thundermaw Hellkite from other colours you are much better off casting a Time Warp and making a six drop
It doesn't have to be a real word from a dictionary

Hans, Run!" Agatha of the Vile Cauldron Alien Invasion Arbalest Engineers Arlinn Kord Arlinn, Embraced by the Moon Arlinn, the Moon's Fury Atalan Jackal Bello, Bard of the Brambles Bitter Work Blaster, Morale Booster Blitz Hellion Bloodbraid Challenger Bloodbraid Elf Boggart Ram-Gang Break Out Brushfire Elemental Cactusfolk Sureshot Charging War Boar Clan Guildmage Deathleaper, Terror Weapon Domri Rade Domri, Chaos Bringer Domri, City Smasher Esper Terra Fanatic of Xenagos Fires of Yavimaya Firewake Sliver Frenzied Arynx Gallia of the Endless Dance Giant Solifuge Giantbaiting Grand Warlord Radha Gruul Spellbreaker Halana and Alena, Partners Impromptu Raid Invigorating Hot Spring Itzquinth, Firstborn of Gishath Killer Instinct Klauth, Unrivaled Ancient Kogla and Yidaro Last Night Together Minsc & Boo, Timeless Heroes Owen Grady, Raptor Trainer Pako, Arcane Retriever Palani's Hatcher Primal Visitation Raging Kavu Rampaging Geoderm Raph & Mikey, Troublemakers Ravager Wurm Regisaur Alpha Revel of the Fallen God Rhino, Barreling Brute Rhythm of the Wild Rip-Clan Crasher Rubblebelt Rioters Ruby, Daring Tracker Rydia, Summoner of Mist Samut, Tyrant Smasher Samut, Vizier of Naktamun Samut, Voice of Dissent Sarkhan Vol Stampede Surfer Toph, Hardheaded Teacher Wildfire Wickerfolk Wolverine, Fierce Fighter Xecau, Predation's Shadow Xenagos, God of Revels Xenagos, the Reveler Yarus, Roar of the Old Gods Zhur-Taa Goblin

Acid Golem is a Rank 3 Xyz Monster thats both easy to Summon (just 2 Level 3 monsters required no restrictions), and packs a heavy punch (3000 Attack Points)
The power level on Giada is extremely pushed