Vikejk17 Arrow is quite dark like Gotham, but it's on season 3 right now.
If you like DC and you are enjoying Gotham so far, you'll probably like Arrow. It starts off slow in season 1, but the second half is great. XD
It's based on Green Arrow of the Justice League, who isn't as famous as Superman, Batman or the Flash.
The Flash is on its first season and is much brighter, but it's another if you know DC, you will love the easter eggs. XD
But otherwise, it's an interesting origin story for him. However, you should watch Arrow's first 2 seasons and then watch The Flash. They're both made by the same people and both their mid-season finales are connected.
Forever is completely different. It's about a Doctor who is immortal. Every time he dies, he disappears and appears in water. XD
I watch Arrow and The Flash. I'm watching Gotham, but it's only okay. Arrow is the best out of all three imo. I also started watching Forever, but I decided to stop because I knew it'd probably be cancelled.