For me, the story and lore unveiled in the sequel makes it a bit better.

They could have used the goop better, but thats a risk when introducing several new mechanics and the goop is the last one

For first playthrough, i like portal 2 better. It also has coop.

For speedrunning and the inventiveness, Portal 1 is just better. Its been broken to such a degree that i myself finished it in under 20 minutes. Portal 2's storytelling and triggers, as well as some reworked exploits just don't allow for that.