I'd recommend starting off with Uncharted, since it's Greatest Hits now. And they have those controller + game bundles for only $5 more than a regular controller, if you need a spare. Maybe get Ratchet & Clank Future: Tools of Destruction as well, if you like action platformers. It's also GH and bundled.
After that, fork up the $$$ for:
- LittleBigPlanet - Fun platformer with endless content, and the best multiplayer game on ps3. My favorite ps3 game.
- Demon's Souls - If you like RPGs, you'll love this. Second favorite ps3 game!
- Uncharted 2 - It's Uncharted, but better. A lot better.
- MGS4 - Assuming you're a Metal Gear fan. It nicely revamps the basic gameplay of the series, streamlining the controls and allowing you to take multiple approaches to each situation (go in guns blazing or never fire a shot), and it's rife with fan service. If you're an MGS fan, you'll love MGS4. If you're not, then you'll probably be far less impressed given how much stuff is tossed in their just for the fans. Oh sorry, it's cliche.
- Killzone 2 - You seem to like Gears, and this feels like Gears in first person in some ways (thoguh no chainsaw =/). Plus it's gorgeous, and the online is addicting once you delve deeply into it.
LBP, DS, MGS4, Uncharted 2, and Killzone 2 are my top five ps3 games, in order. I honestly don't think you could go wrong with any of these, though keep in mind that a majority of the enjoyment from Killzone comes from the multiplayer, and it's not very jump-in/jump-out friendly. You will have to sink some time into it.
Also, I happen to have a spare, sealed copy of Valkyria Chronicles lying around for a rainy day... >_>
