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Now as I weed through them I'm trying to decide what to keep and what to return >>
cthd:
Obviously, don't keep any of the cancelled games
Seriously, I've found Gamespot's reviews to be quite accurate as far as DC titles (which is not the case with some of their reviews for other platforms).
You could take a couple of approaches. Probably the best is to decide on a certain number, and return anything that was rated lower than that. A good score to start thinking about as a minimum cutoff is probably 6 ~ 6.5.
Personally, I wouldn't keep anything lower than a 6 at all, UNLESS there was some nostalgia thing for a game's subject matter. Even then, I might have to think about it, because a really BAD flaw (control issues, unbalanced gameplay, dead stupid or deadly unfair AI) makes for an unplayable game. And an unplayable game is dead weight in yoru collection.
I plan to keep the games which maximize my investment in DC controllers, like the VOOT Twin Stick, fishing rod, MC2, and arcade controller.
Also, I don't plan to keep any sports games, since I'm not into them.
As far as collecting items goes:
Official maracas and Samba de Amigo
Twin Stick and VOOT
Dance pad (not mat) and DDR
TXR2
Gundam Side Story
Marvel v. Capcom 2
HALF-LIFE??????? (this was never released, was it?)
Gundam Battle Online
As far as excellence:
Soul Calibur (the DC's killer app)
Jet Grind Radio (arguably, the 2nd killer app)
DOA2
F355
Marvel v. Capcom (both)
TDLM
aw heck, anything off the list below rated 8.5 or higher...
Just MHO