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in no particular order, it's harder to pick than a broken nose but my own challenges: the stellar swan dive rescue the runway start-stop land speed record the grand tour (AKA The Kessel Run)

Mine looked like this: Yep. I say mine looks better.

I built a scale replica of Spacedock from Star Trek once... over 2500 parts, it nearly killed a quad core.

I'll get 1.3 running eventually and reboot it, hopefully before v2.0 comes out!

the runway speedrun challenges (shameless self-plug) should be on here, as they spawned a couple Ironman challenge entries on Youtube and was so Kerbal, even Scott Manley went "huh?". Links to follow, but the basic premise was: go as fast as you can on the runway and stop before you reach the end. It's insanely difficult to reach Mach 1 and *stop* in 2km without your wheels (or skids!) leaving the ground and without breaking your vehicle, but a few managed it!

I have a working model of the Su-37 Berkut for 1.1.x, I'll update and test for 1.2.x (but it won't have Mechjeb since I can't get that working in 1.2.x), it'll be cannon only - because it doesn't need missiles.

I did something similar a while back as a storyline challenge, managed to not only rescue all the kerbals from a runaway station on a death dive into the sun, I also managed to use a Munar gravity assist to return to Kerbin - without using heatshields.

Probably not... I'd defer to the OP though, this ain't my challenge.

So in case you missed it, the Ekranoplan is a Soviet-era ground effect vehicle designed to go very fast at very low altitudes with large cargoes. Originally conceived as a weapon delivery system, the Soviets realised early on that missile delivery was far more cost effective, so the Ekranoplan was relegated to moving troops and equipment in the Black Sea until the program was cancelled in 1985. These days the Ekranoplan concern builds vehicles for civilian use. Note on the above prototype: Super-stable "Ekranoplan" cargo platform. VERY nose-heavy, which is perfect with the huge amount of lift which means it goes just barely subsonic, at an indicated altitude of just 20 metres, with throttle at 15%! This thing went right around the planet (land and sea) in a little over 3.5 game-hours (with the odd diversion to avoid mountain ranges!). The only problem with a water landing meant that with it being so nose-heavy, it pitched down into the water with the slightest thrust, so it couldn't take off again. I had to slowboat it (after a water landing!) back to KSC, which took about 20 minutes. This prototype consists of a long cargo bay and two opening tail sections, and several airbreathing jets. Maximum payload length: whatever'll fit in the MK.3 CRG-100. Oh. Right. This is a challenge page. OK. Here it is: build an Ekranoplan with a surface-deployable cargo (it can be a wheeled vehicle, pop-up rocket launcher, or a narrow-winged fixed wing aircraft or rotary wing), "fly" the Ekranoplan to the Island runway and *deploy the payload there*. Then "fly" the Ekranoplan back to the KSC shoreline and make a water landing. RULES: - Ekranoplan must be crewed by at least 1 Kerbal - Ekranoplan must be internally fueled (no taking fuel from the payload) - Payload must be functional and self-contained (power, comms, control and fuel) - Cruise altitude over water may NOT exceed 20m. Usual rules about bits breaking off, physics/exploit cheats etc., apply.

It is an old challenge, it was one of mine and it spawned a couple video series (and the Reddit Challenge!). I'll edit this post and add the link once I find it. (EDIT: there are TWO legs to my challenge, one using the OLD aero model, the other using the new, both have a pretty comprehensive set of rules):

OK, apart from the winglets and the multiple SRBs (not having the extra large ones in my module stock), this is essentially identical to the American space shuttle assembly. Flight Profile Start on the runway, at 0 roll. Throttles to max. Shut off fuel flow in the orbiter tank and the nose monopropellant tank. Immediately upon liftoff, set pitch at 60 and roll at 180. Toggle Q and Vt off! The vehicle will try and pitch vertical: this is the reason for rolling 180 - it offers better aerodynamic control, putting the centre of mass above the centre of lift and keeping it there. SRB shutoff at T+64. Set ME throttle to 10%, SRB sep at T+65, throttle to 100% at T+68 and set pitch to 0. With the new thrust vector, the craft will pitch 0. When the oxidiser in the ET runs out, the engines will shut down. Pump remaining LF into the orbiter and roll to 0, then jettison the tank. Deactivate the three main engines: orbital insertion will be performed using the small LFO OMS engines which vector through the orbiter CoM. Once in orbit (I simply circularised at apoapse), deploy mission parameters. In this case, it's a low orbit communications satellite. Note the spacelab module in the orbiter offering more accomodation, a docking port and sensor package. I haven't stuck landing yet, after 37 tries! (EDIT: Try #38, I got a safe water landing, only the ventral OMS engine broke off!)

The Big Round Thing is still up there in its incomplete state (might finish it this weekend), but here's another I sent up in one piece because I got a bit impatient with launching and rendezvousing the same shape payload another half a dozen times... As you can see, it's about as round as a bicycle wheel having just gone over a tram track. Suggestions for a name welcome.

oh my bad, it is on escape, but not from launch. No rocket that's ever existed could throw it that hard*. But a gravity assist from Jupiter could. *unless you trip the Infinite Fuel cheat.

I made a Borg cube, back on the pre-0.24 days...

dangit, I was going with a Teal'c Torus.