The better mech series?
That's very hard to say.
I'm going to presume you meant Macross when you said Robotech, because that allows us to bring in so much more stuff (and exclude Southern Cross and Mospeada).
Battletech is a fantastic game with a good setting, but even as a fan of the more, less-aesthetically pleasing mechs, it suffers from some poor designs and miniature quality. Also, while the rules quite literally allow you to do anything in game, many people are put off by the record sheets or don't want to bother with less detailed versions. The amount of computer games put out are stunning and while there are one or two bad ones, it helped to define an entire genre, one that's on the up and coming. Also, the cartoon, while mostly crap, had a magnificient ending. Maybe not worth watching the whole series for 2 minutes of enjoyment, but it was definitely one for the book.
Heavy Gear's rule system is brilliant and DP9's Silcore system is also a great system. The plot and universe of Heavy Gear is also one of my favourite and the miniatures are some of the best I've ever seen on a tabletop. It's also had two computer games, which are some of my favourite in the Mech piloting genre. Its animated series however, was not great and didn't even have any remarkable suprises that redeemed BT's to me. Also, the cost of the miniatures is incredible. I've only got two squads of 5 Gears and that still put a dent in my wallet enough that I could have picked up an entire 10mm batallion for BKC.
Mobile Suit Gundam: It's huge, it's rightly cited as introducing the concept of robots as military pieces of hardware, and I love it dearly. That said, it's not perfect. Part of the charm of the original series is the 'quality animation'. Also, Tomino does some interesting work as a director. When he's good, he's very good, but when he's bad... well, I enjoyed ZZ Gundam once they got to Earth. Also, as a Feddie, the lack of time we get compared to other factions doing cool things could probably fit in a five minute clip in the entirety of the collection of animation. That said, there's so much there that you're bound to find something, from the crazy-as-hell Imagawa directed G Gundam to CG Federation Infantry taking on Zakus with ATGMs and even when you're annoyed at the plot, they put a lot of effort into animation so at least it looks awesome. I will add that while I could get through nearly every Gundam series with at least some enjoyment, SEED and Destiny just turned me off. They're not for me, and I could go on this tangent in some detail, but my post is pretty long already.
There's so many games out for Gundam, it's staggering. Unfortunately, most of the good ones don't come out over here (Gihren's Greed, for instance). Also, a word on the models. Bandai has supported Gundam with models for a long time and recently, that's paid off big time. Bandai, in my opinion, makes some of the best model kits known to man; they fit together brilliantly, they hardly need any work seam hiding, the moulding is crisp and lacks flash. While mech models might not be to your taste, technically, they are the best model kits I've ever come across.
Macross is the one I've had the least experience with, I'm afraid. though I'm currently watching SDF Macross. It's got some interesting plot elements, but I'm a guy who likes his mil-sci-fi to be between humans, so it starts out not my cup of tea. That said, there's a ton of content there; three TV series, SDF Macross, Macross 7, Macross Frontier, DYRL which is one of the best looking pieces of animation I have ever seen and a host of OVAs all of which are great fun and even when the plot fails, use some interesting ideas and at least look stunning (at least the ones I've seen). There's also a whole host of games, many of which look very cool, but unfortunately remain in Japan. The designs are fantastic, though model kits vary on price and quality. I'm currently making several aircraft kits to practice for a Hasegawa VF-1D kit I've got in the cupboard. SDF Macross also suffers from 'quality animation' as well.
EDIT: SPOILERS: Alright, technically not quite interspecies warfare, but still, enough differences.SPOILERS END
Anyway, my favourite series in regards to Mecha? Patlabor, Dougram and AT VOTOMS, with BT and HG as my favourite mecha wargames.