I'm guessing a large crowd could argue that it's the Hover. It's ability is probably the most enjoyable for anyone who explores or escapes maps, and it's the only robot in the game capable of flying in all directions. It was also the very first robot to be able to fly forever. It's really useful for glitches such as escaping or just fooling around in general like trying to carry someone, although it's flight speed could use some work.
Although I love my own Hover and have had it for 5 years, I know there are a lot more fun robots I have never really had the ability to try.
Although it's not really an old robot, just about every dash bot including the Nether is generally a fun robot. They have fun gameplay in terms of dodging or catching someone off guard. Personally although I have a Nether, I have always wanted a Kuhimo. I think the 5 second reload and generally short dashes would be fun to play with. I should also note that dash bots are good at glitches as well; especially the Nether as it's able to cross an entire map or escape using sheer speed. You can also accelerate another player if they are in the air and you hit them while dashing.
I know this is a bit of a summary of groups but the Pantheon, Dragons, and Shapeshifters are all fun or unique robots to play as. Maybe not not because of some difference in technology but throughout history they have been an interesting bunch. For me, although I have an Ares I would say that the Nemesis would be my favorite Pantheon, in place of my Ao Jun I think the Ao Qin is cooler, and I never had any shapeshifters but a Tyr in my personal opinion has the coolest playstyle.