What was the first warframe you "mained"?

I've settled on a weapon to help me through the star chart (I chose the Phantasma, if anyone's wondering) and it's been going pretty well, but I've been wondering when you should decide to invest a reactor and endo in a warframe. I've just been using Rhino mostly due to his tankiness but his kit is pretty boring so I don't think I'll ever invest heavily into him.

I don't find myself dying very much so do I even need to both investing into a warframe right now? Or should I just keep unlocking them for the time being and trying them out?

But yeah, what frame did you guys decide to properly kit out?