Could be from Gundam, or from any Mecha franchise. For me from Gundam, I think Barbatos is my favorite, I love the sleek animalistic look. But overall, I love the Armored Core mechs, I think my favorite design style is what that series hits with a cross between human looking and machine designs. I also love the artwork for Lancer, I think a lot of mech designs in that game are really evocative, and I really want to get a game going if I can get my more fantasy oriented friends to play something other than dnd5e, lol. Also, special shoutout to any mech with wings
The one that got me I to Gundam was Wing Zero from Endless Waltz, so I have a huge soft spot for that one. Other ones I really love are Unicorn, Barbatos, and GP-01.
Outside Of Gundam, I like Laevatein from Full Metal Panic, the designs of the Flight Units from Nier Automata, and GaoGaiGar.
Really looking forward to Armored Core 6!