Back in the saddle again!
Mech or Mecha series include not just the classic giant robots but smaller "mechs", ranging from the hardsuits of Bubblegum Crisis to the labors of Patlabor. This means that there's a WIDE range for "favorite mech" to cover.
But for me there's really no competition at all, because the answer here has always been battling it out with Ah! Megamisama! for top spot on my "Favorite OVA Series" list: GIANT ROBO: The Day The Earth Stood Still.
This marvelous retro-future SF has it all: giant robots of baroque and ludicrous designs, Mad Science!, superpowered spies, shadowy conspiracies swearing Allegiance or Death! to their goal of total world domination, magnificent heroism, and a soundtrack that shakes the room. The saga of Daisaku Kusama, the young boy in whose hands rests the control of Giant Robo, the most powerful and mysterious robot in the world, spans the heights of triumph and the depths of tragedy and despair on a Wagnerian scale. Beside it, other mecha series are... well, MUNDANE. The Gunbuster is infinitely more powerful, Patlabor infinitely more realistic and believable, Gundam infinitely more influential, but none of them can compare in poetry or power to Giant Robo.
"Giant Robo -- CRUSH!"
Day 1 - Very first anime:Saint Seiya
Day 2 - Favourite TV series: : The Vision of Escaflowne
Day 3 - Your anime crush: Belldandy, from Ah! Megamisama/Ah/Oh My Goddess! and Shun from Saint Seiya
Day 4 - Favourite Film: Tonari no Totoro/ My Neighbor Totoro
Day 5 - Anime character you feel you are most like (or wish you were): Shikamaru from Naruto Shippuden
Day 6 - Most annoying anime character: Mihoshi, Tenchi Muyo!
Day 7 - Character you’d most like to slap: Sasuke, Naruto Shippuden
Day 8 - Most epic scene ever: End battle of YuuYuu Hakusho: The Poltergeist Report Movie
Day 9 - Saddest anime scene: Belldandy erasing herself from Keiichi's life in Ah! Megamisama/Ah/Oh My Goddess!
Day 10 - Favorite slice of life anime: Koko wa Greenwood
Day 11 - Favorite mech series: Giant Robo: The Day The Earth Stood Still
Day 12 - Best instalment of a franchise you don’t like
Day 13 - Worst instalment of a franchise you do like.
Day 14 - Current (or most recent) anime wallpaper
Day 15 - A reality check for a genre you like
Day 16 - An anime classic that simply doesn’t work for you.
Day 17 - Best Background Music
Day 18 - Best Five-team (?)
Day 19 - Change you’d like to make to any one anime
Day 20 - Most ambiguous anime character
Day 21 – Favourite OAV Series
Day 22 - Animation studio you'd most like to slap.
Day 23 - Anime you think had the best, or most intriguing art
Day 24 - Favorite anime hero or heroine
Day 25 - Best anime villain
Day 26 - Your favorite harem anime
Day 27 - Favorite anime opening theme song
Day 28 - Favorite anime companion/summoned creature
Day 29 - Best Anime Music Video
Day 30 - Favorite anime ending theme