A Chevy small block is typically the best choice for low cost, high performance, low weight, and ease of installation. A 350 is the most common choice, but if you want more torque there's also 383's (a stroked and bored 350) and 400's (the biggest small block GM made). If you want to keep it all Buick, I've actually seen 455 Buick big blocks in 80's Regals. Probably requires a lot of mods though.
It really depends on what kind of performance you want and your goals for the vehicle.
