Galaxy was fun but I didn't like the fact that your extra lives erased upon a reset and the powerups were only found in their set levels. They shoulda spread the power ups to more levels to make it interesting and added more hidden stars, and not erased your extra men (defeats the purpose to explore and collect extra men). You can find it for $39 if you look hard enough.
Paper Mario can be fun if you liked the paper mario series. It needed more action and less talking for me (it was my first paper mario game). It was a pretty lengthy game though. I played it about the same time I beat Galaxy and only got through a few worlds.
Strikers is a fun multiplayer game. I played it for a while until I lost quickly lost interest. The teams are extremely uneven and certain teammate combinations are unstoppable. The game felt easy and repetitive after a few games because characters do the same moves over and over. Online is the same, people use the same characters over and over and over again. Strikers is fun, but felt like it would be best if it was $29 dollars.
I have heard mixed opinions about Mario Party in the fact that the Wii motion controls feel "tacked on". My 7 year old Niece has it... she says it sucks. She also says Galaxy sucks so... No comment on Party 8.
Good thing you can rent games.
I think they should make a new Mario game that has the 2D/3D action of Paper Mario, but make it a straight platformer similar to that of the DS. Include the powerups from Mario Galaxy and the DS versions.