I'm not a mecha fan. As much as I love Super Sentai, I hate that mecha's made to be such a big deal. I grew up with the mid and late '80s Sentai shows, where mecha battles were short and sweet. Those shows treated mecha like the tools they are, not a the-world-circles-around-them importance to write entire episodes -- even entire shows -- around.
My favorite united mecha is Flash King. My favorite individual mecha is Liveman's Land Lion. I can't really think of a mecha fight I love off hand -- it's more about the emotions of the team members going into a particular fight that will stand out. The Changeman facing death as they take on Gozma Star, the Maskman endangering themselves and braving attacks trying to sync their aura up with Land Galaxy's. The GoGoFive risking themselves to nuke an asteroid, with Mondo thinking he loses his kids. Fight scenes themselves are pretty limited with mecha. I can think of a lot of cool finishers or moves, but not really a fight that sticks out the way the human-sized fights do.
Despite not being a mecha fan and disliking shows that heavily feature mecha, I love what GoGoFive does with its mecha. It can be a bit too mecha heavy, but they get back to that feeling of the mecha being a tool. This show, more than most, uses its mecha in different and interesting ways -- in rescue missions, in travels to space to stop a dangerous asteroid. The miniature and effects work in the show are great, and go into the right amount of detail for you to care.
Modern mecha is terrible, though. While there have been some mecha I've thought haven't looked too bad since (Shinkenou, for one), the last mecha I truly didn't mind was Time Robo. I thought it was interesting that you could get two completely distinct, full on Robos out of the same design. I miss when Bandai put effort into the designs, getting the individual pieces to actually transform and form an actual design that looks pleasant. Ever since Gaoranger, I don't understand why, they've just been lazy with just slapping the complete individual pieces together in one ugly blob -- there's no real transformation anymore. That's when it became, like...instead of Gao Shark folding up and looking like an actual arm for Gao King, the entire Gao Shark is just slapped onto the side of Gao King's body, and it will just have a big shark arm with a shark head fist. It's dumb! It's lazy. And these are people being paid to come up with something so dumb and lazy.