Usually, people tend to put either episode 2 or 3 in the number 1 spot when it comes to the Saints Row series. Personally, I prefer The Third but whichever episode you played most one thing is for sure: it all started with Saints Row 2 on PC. The first part has always been Xbox exclusive. I wish this one got a Remastered version! There is a compilation of its cutscenes on YouTube and the videos still look great.
Why play it now? I had a quick glance at the latest (and last) part and it was too bright for me. Not sure why they ditched a lot of things and decided to take the reboot route. What about Gat out of Hell? Meh... it was always a bit of a mystery for me so I never played it. For me Saints Row is part 3 and 4 (yes, the idea of a Matrix-like version fascinated me).
Now, this one picks up the story where it was left off. The gang is still in Stilwater - Steelport was only introduced later as the main setting. I can imagine people fell in love with part 2 immediately: it already had realistic physics (the graphics were not as next-gen as in GTA IV that was released in the same year but this was definitely ahead San Andreas in terms of technology).
We saw full character customisation for the first time and picking different gender and race meant different voice actors (altogether 6 of them if I'm not mistaken). If you started with The Third then you never got mad at Volition for getting rid of Gat who appears to be the first person you recruit as you get ouf of prison. Then you continue with Shaundi and Pierce later on. One hour in you already have lots of missions and activities you can choose from and 2 cribs.
While Part 2 is still not as serious as any GTA (essentially a trademark of the series) it isn't as crazy as The Third became later on. If you don't mind the dated graphics and that the FPS value varies between 40 and 60 on the highest settings (for no apparent reason) then pick this one up. Ditch the reboot!