Mazinkaiser SKL, All Getter OVAs (Shin vs Neo, Armageddon and New Getter) and Dai-Guard (pretty obscure super robot spoof with salary man as the pilot) are among my favorites, but the others I've watched throughout the year were also pretty good too.
The others I've watched so far are:
Getter Robo Arc (very divided on it, but i loved the remixed OVA soundtracks and callbacks it had)
Shin Mazinger (the plot is a big ass mess, but the OP, action and mazinger itself is badass as hell)
Mazinkaiser/Mazinkaiser Vs The Great General of Darkness
Kotetsushin Jeeg
The Big O (season 1 is perfection, i dont like season 2 and how it tried to be "meta", it ruined the immersion for me)
Mazinger Z: Infinity
Megas XLR (i know this isnt a mecha anime, but still falls into the genre)
Linebarrels of Iron (i hear the manga is better, but i did really like the mecha action and linebarrel's design but i can really see the glaring issues it has)
I was gonna check out Brave Bang Bravern! but the fag undertones and gay shit, even if its played for laughs really turned me off; the only other one I'm waiting to watch is Grendizer U, which can either be a big ass mess or really good.