Good idea @aBc (though it's usually best practice to provide your own list first when soliciting the lists of others!). Here's my top ten list in order of nostalgia:
1. Myth II - I forget if it was from the first one or the sequel, but it was the first time a tutorial made me laugh out loud with the line "Press the Space Bar, the big key below all the little keys!". Really fun squad-based RTS and LAN-capable!
2. Bolo - As my username suggests, I'm a big Bolo fan from back in the day. It was astounding how well it networked on machines that were ancient in the late 90s (over serial AppleTalk no less), and building forts out of turrets only to have the rest of the players gang up was some great emergent gameplay. LAN-capable too!
3. Marathon 2 - It was *our* first person shooter, made for the Mac back when Bungie was the best. While not as flashy as Duke Nukem or gory as Doom, Marathon was eerie and puzzling. And I still can hear the gun and explosion sound effects from the first level and the voices of your comrades as they teleport away... "Get me outta here!". LAN-capable!
4. Civilization 2 - The game that needs no introduction and is one of the finest iterations of the series. The videos of the advisors were the coolest feature back then, especially when they'd agree or disagree with each other's advice (lookin' at you, Luxuries Elvis). LAN-capable!
5. The Maxis Simulators - It's almost unfair to include the Sim City, Sim Tower, Sim Ant, and Sim Farm games in the list, but we were so thankful back in the day that Maxis ported their games to the Mac. One of my earliest gaming memories was my parents finally relenting and buying me Sim City 2000 from Egghead Software. I still remember walking out into the cold, crisp night with the fresh scent of petricor, opening that big ol' box and pulling out the thick manual that would be my de facto bathroom reading for the next couple of years.
6. The LucasArts Point-And-Clicks - I could fill the top ten with just those games, Secret Of Monkey Island, Day Of The Tentacle, Sam And Max, Full Throttle, Loom, Secret Of Monkey Island 2... the list goes on.
7. Majesty - It's good to be the king! This was the first "idle" game I played and I really enjoyed how relaxing the gameplay was. Sure, half the kingdom's on fire and the sewer rats have teamed up with the roaming dragons, but you pay other people to take care of that! Just apply a bounty and make sure your heroes can buy healing potions and sufficient armor at the shops, and the kingdom practically runs itself! It's LAN-capable too!
8. EV Nova - The culmination of Ambrosia's Escape Velocity series, a vast universe with a rich storyline to get lost in. It's difficult and unforgiving, but once you get the hang of combat (and a ship capable of withstanding some damage), the power fantasy kicks in and you can start to boss around whole planets.
9. Maelstrom - Before there was EV Nova, there was Maelstrom. Simple. Addictive. Timeless. Smash up those asteroids and collect those powerups for as long as you can!
10. Spaceward Ho - What EV Nova is to action adventure, Spaceward Ho is to strategy and 4x. The "hmm" sound effect for finding a mediocre planet is burned into my brain such that it's what I hear whenever I'm mildly disappointed.
I'm excited to see others' responses! I would have put Warcraft I, II, and Starcraft on this list but those are the obvious choices and I wanted to spotlight some of my more obscure favorites.