These days C is the universal language supported by every microcontroller on the market, or C++(ish) used by the Arduino. Once you learn C it hardly matters what hardware you use although the quality of the toolchain does vary widely. I'm still using 8 bit AVRs since they're the devil I know, but ARM such as the popular STM32 seems to be the future.
I've come to quite like the Arduino too, I was not impressed for a long time, the software is fairly clunky but what finally sold me on it is a combination of the proliferation of insanely cheap clones and the massive collection of libraries floating around that make interfacing to all manner of random poorly documented Chinese widgets absurdly easy. I can pick an LCD or OLED display out of my box of China gadgets and in literally 5 minutes have it doing something. No spending hours pouring over blurry Chinglish datasheets trying to decipher the commands needed to wake it up, just grab the library, load a demo sketch, hook it up and go. Additionally it now supports a collection of other boards like the ESP8266, ESP32 and some of the ARM boards, I ported one of my earlier Arduino projects to the ESP8266 in just a few minutes, same IDE, same code, just change the IO pin names to suit.