The Hollows.
I especially liked the Hollows during the early period just after launch. This was when whole spawn groups of Trolls or Outcasts would periodically rush the area around the Atlas gate. It was a very exciting time. I was really disappointed when that got turned off.

The Hollows was where I took my Empathy/Energy Defender out of retirement. He was my second "main" character, but I had to put him on the shelf some months prior when I realized that I really sucked at being an Emp. I had gotten him to lvl 30 or 31 during a few weeks of heavy teaming. I was still pretty newbish, so I didn't notice how much the team had carried me along. Then I joined a PUG Manticore TF and it was a disaster! I couldn't keep up with all the damage and the team completely fell apart during the first mission, ie just about everyone bailed. Looking back at it, I think the problem was that most of the team were lowbies for the TF, so everything was red to deep purple (back when the enemies of a TF were always at the high end of the TF's range) and I was using mostly DOs still.
So anyway, one day while hanging out at The Hollows (probably its first week) with a new character, I saw an Emp player had positioned himself in the middle of the parking lot in front of the Atlas gate where the Trolls and Outcast would run through to get at the NPC defending the gate. The guy had set his Healing Aura on auto and went afk. He was basically a healing station. I made use of him. I was able to practice pulling from a large spawn group with the safety of running back to him to fight them all off if necessary. This guy's afk healing was cheesy, but it still impressed me and inspired me to bring out my own Emp to help too.
I started just healing the lowbies around the gate during times when I was bored and didn't feel like playing my other characters. Then I ventured out further to cover those on the Cherry Hills. During this time, my brother told me about the option setting to Show Health Bars and it was an epiphany!

All this time I was statusing player health by mousing over their characters. With Show Health Bars = Always, every player character on my screen had health bars over their heads. I could monitor dozens of people simultaneously. It was amazing.
My Emp spent (truly) a year and a half training in The Hollows, using 2 respec as I rethought my build and play style. I got to the point where I could cover 3-4 separate teams spread out across a 6-8 block diameter fairly well using mostly Heal Other, Fortitude, and Clear Mind while Flying and the occasional Superspeed along with remembering and anticipating where they were each heading. I had also been tweaking my own combat capability and was able to gain a couple of levels soloing my missions, something I hadn't been able to do since the low teens. After a 3rd respec to refine things, I officially took my Emp "out of The Hollows" to join teams and really did know what I was doing.