Deathstroke, Reverse Flash, Captain Cold.
A glimpse of DC's favorite and baddest villains to wreak havoc for Arrow and our Scarlett Speedster since the DC Television Universe started to pave the way for Superhero shows! Captain Cold (Leonard Snart) is hands down my favorite villain that is crossing over on Legends of Tomorrow which is really excellent! Deathstroke (Slade Wilson) a dark man of unbelievable force is known for his battles with Batman and also the Teen Titans! Hopefully there is a show for that soon and maybe it crosses over with Arrow and The Flash?! Reverse Flash. A speedster of pure tyranny that kills Barry Allen's mother devastating his life and becomes an ultimate foe against The Flash. All wonderful villains. All capable of sucking me into the developing world of Barry and Oliver's even further, with a small occurrence of crossovers, Multiverses! Suicide Squad formation, and Heroes such as Hawkman, Hawkgirl, Heatwave, Firestorm, White Canary, villain additions as Grodd, King Shark! (Barely) Bronze Tiger and many other characters from paper.
Of course one of the treasures from this Television Universe was the small jail cell of Harley Quinn.... Who could forget that chill when seeing those letters?