For me, Mass Effect is a major game for me. The first game (which I still played) I sunk over 80 hours into it per character (and i've done 3, soldier, engineer, adept) and found something new every single time I played. I really loved the idea that it would cross into the sequel but more than that loved the idea that the achievements (xbox version) actually affected your later playthroughs. I wish they'd kept that for number two because it aided replayability.
Numer two I've only played through a few times, but am still enjoying the hell out of it. Love the two main addons for ME2. Shadow Broker was awesome and have yet to play through all of The Arrival, as I started it before I'd finished a playthrough and want to do it after I win one of my playthroughs.
ME1 was citadel space.
ME2 was Terminus system.
ME3 needs to be both.
Also, does anyone else think Wrex's asari friend/enemy mentioned in number one is going to be revealed to be Aria T'Loak?