I've got too many to list in full since I've been playing online for years, but I'll dig up whichever ones I remember.
I had some great games on AOE3. One of my weirdest was a 3v3 game where both sides rushed each other and both sides had to evacuate villagers into the land their soldiers had cleared to rebuild everything. Essentially, we just ended up switching sides and the match continued.
Another good set of games was where I played somebody on my friends list who I thought was a bit of an arse. I think he only played against people he knew he could beat so he could get his score up higher. He had invited me to stacked team games before, so that's why I thought that. Anyway, he went Aztec and I went Iroqouis (sp?). He went for a rush as most Aztec players do, and I had to throw myself into holding out so I could get into the fortress age. When I eventually managed it, it was a great feeling. Seeing my (gun wielding) forest prowlers scatter his (sling and spear wielding) crappy troops was amazing. He resigned soon afterwards.
He challenged me to a second game, and to counter my strategy he went with France, who are a little stronger in the fortress age. So... I tomahawk rushed him perfectly and he resigned again. I guess I paid him back for that loss he inflicted on me with those stacked teams earlier.
On C&C3, one game stands out for me. It was when I was playing with a real life friend who hardly played. We were NOD/GDI vs a Scrin/Scrin team, so I called that we should go for a rush. My side of the rush fails miserably, but my friend's doesn't and he manages to cripple his player. Sadly, since I messed up, I'm in a weak position, and the guy I rushed is in a strong one. I have a small outpost in the middle, and I lock it down with all the anti-air defences I can build, getting ready for a scrin counter-attack. His big planetery assault carriers roll around, and the battle that follows is epic. All those tiny ships flying around and being shot down by hundreds of missiles and bullets is just awesome. Eventually, I manage to shoot down all his little ships, but his crippled team mate comes at me with tons of crappy tanks and... ore collectors. Since the Scrin ones blow up, he uses them as suicide bombers, and the two together force me out my outpost. I fall back to my base, and just when I look doomed, I find out what my team mate has been doing while I've been defending. Building tons of stealth tanks. If seeing all those little ships fall out the air was cool, then seeing all the assault carriers fall out the air is even better! After this, we counter-attack and win. That was a really tough but satisfying game.