My favourite accomplishments mostly center around rendezvous, and I think I learned the most when simulating the Apollo X mission. It became something of an obsession at the time because I figured that not landing on the Moon would be harder because most mods/addons are based on a lunar landing and I wasn't quite sure how the simulated AGCs and autopilots would handle not landing.
Anyways, that was the one where I learned the most about phasing burns and the Concentric Rendezvous Sequence, and which set me up nicely for Apollo-Skylab, Apollo-Soyuz, and Gemini VII/VIa and X missions. It was also the one where I realized that NASA presskits have things doped out pretty well if you're going by the numbers and looking for a flight plan to work (flexibly) off of.
Now, the Gemini X dual-rendezvous mission became my most recent obsession in Orbiter because I wanted to see exactly how little fuel-usage I could be successful with, my main concern being the minimization of delta-V during Terminal Phase Initiation, which dropped to ~8mps during what I call my "successful simulation". The first time, when I was using a more "direct" rendezvous, I wound up with an unacceptable ~35% fuel onboard the s/c after the first rendezvous. After working out the kinks (i.e., thinking too much) and concentrating on a "by the numbers" concentric rendezvous, I came in on a TPI tangent (that 8mps) from Constant Delta Height, and was satisfied with ~75% fuel remaining after the initial rendezvous; and after the second rendezvous and deboost, 50% remaining. Hell, fuel-usage could probably be brought down even more, but it was certainly satisfying.
One thing I really enjoy about Orbiter is perfecting a maneuver and seeing mission goals being ticked off the checklist on-time and well inside any restrictions. It's tough to find time to enjoy the view during these goal-oriented flights, but that's what joyrides and EVA are for.
See ya at splashdown, fellas! :beach: