Uncharted 2, definately. Both brilliant games and franchises but...
ME1 was already a very good game which was let down on the technical side (long loading/elevators and relatively poor gunplay) whereas Uncharted was good but otherwise fairly average compared to other games on the market.
U2 added multiplayer, co-op, better graphics, a proper stealth mechanic, better pacing and generally more polish all round.
ME2 improved the gunplay and refined the experience but as far as I was concerned the base experience remained the same.