CertainlyT was unironically one of the greatest holistic Designers (not just champ designer) that League has had. I know this is going to get memed because people will say "yeah he just taught people how to make broken stuff or how to balance worse", but I don't think he did those things. Designers here are definitely good enough to have their own opinion and be able to defend it. He definitely pushed the boundaries in a way that is not everyone's cup of tea, but that's part of what makes him great. He also knew how to make simple champions (think WW update).
It's unfortunate, because players will never get to see behind the scenes of how he contributed, but one of his best attributes was that he truly passionately cares about League. He was always up to date with all the new builds (often as soon as they were popping up in KR soloQ) and he knew all the little details about the game inside-out. In his spare time, he was always prototyping stuff locally on his computer to make League better, whether it was little champion updates, new champions, ways to make spell queueing better, investigations on how to put in latency protections for champion combos, making sure unpopular/weak champions were thought about, etc.
He was amazing at pushing designers to think outside the box, to challenge their assumptions about their own world view, to have a good design process, the values of elegance, intuitiveness, feel, how a player failing on a champion leads on a path to mastery, even the purpose of games and how people consume them. Outside of that, he was an amazing mentor and would help you work through your design problems for hours after work (not just straight up giving you the solution, but helping you work through the problem the way a teacher would; unsurprising given his teaching background), often until 2am (I'm pretty sure he didn't sleep). He did have strong opinions, but that's par for the course for many accomplished people in their field.
Even outside of League, his designs were elegant and powerful (eg. TFT item system). It always pains me when people bash on CertainlyT in this sub (though of course you can have your opinion), but I think he has unironically done so much good for League over the years (at least for ~3 years for when our tenure has overlapped, his entire life was basically dedicated to making League better from waking to sleeping) and for that I'm eternally thankful.
EDIT: He's not dead, he just doesn't work on League anymore, so we don't see him as often :D