There only can be one answer to this and it is of course Rolemaster. The great injustice is of course the name calling, chartmaster, rulemaster and so on. The total failure of the greater RPG community to differentiate between all the optional rules that are available and the actual rules one needs to play.
And lets face it, compared to Pathfinder Rolemaster is positively lightweight.
Rolemaster always said it considered itself an ‘advanced’ system. You shouldn’t grab the Advanced books if you want something simple. I know I did but I was hooked on RM and then wanted something simple. It is just a testament to the adaptability of Rolemaster that actually I could carve out my simplified, rules light Rolemaster and have it work seamlessly with all the standard monster stats and the few RM native adventures I have.
Customisation does not have to mean complexity. The core RM structure is so robust that you can hack it any way you want and it remains recognisably Rolemaster.
What Rolemaster needs is for the community to honestly hold up Pathfinder or D&D (in any of its recent incarnations) in one hand and Rolemaster in the other give them a fair comparison. Drop the prejudices formed 40 years ago and look with fresh and more mature eyes.
Rolemaster is not FATE and it never will be. If you lean that way then RM is not for you but RM is, in my opinion, more robust than D&D 5e and more than capable of being tailored to your style of play whatever that style of play is. What ever you want to do with it, there will be an option for it. Just pick the options that you need and no more.