I Only just recently bought the last 19 songs I was missing in my collection (so now I have EVERYTHING), but from what I've played enjoyed DLC wise off the top of my head...
Tribe Called Quest - "Scenario", I Don't know how this song got a 3 or 4 dot rating, it definitely feels like a 5 or a 6. Regardless the song is TIRING, because it's not edited all the way down like some songs. Even with that, it's fun and it throws a ton of moves at you, but none of them feel overly complex to nail down.
Timbaland f/ Keri Hilson and DOE - "The Way I Are", this is a Dance Central 1 piece of DLC (meaning forced Freestyles), but I still enjoy it alot. Learning the moves on this one are alot more strict (as is alot of DC1 songs compared to 2), but once not impossible once you notice the small things here and there. That and there are some original moves and the beat is fantastic. Also being able to nail the timing down of the last couple of moves is a fantastic feeling.
Rihanna - "Disturbia". Good warm up song, it strikes that point for me where it looks a bit complex, but is a simple dance that is repetitive, but doesn't look it when you see someone play it. Another DC1 era piece of DLC.
Keri Hilson - "Turnin Me On". I Think that right point of feminme dance moves, without looking too weird for a guy. DC1 era as well.
Cee Lo Green - "Forget You". It's a song everyone knows and the dance even on highest difficulty isn't too bad for a group (thanks to some repetition) to dance along to, if not then just to laugh at some of the whining moves.