UAD by far IMO. Waves got a rep for "quality" because of their "quantity" and accessability. They're highly bottlegged as the "top line" solution for audio. They have good plug ins that drown amidst the hundreds of modules they offer. IMO there's plugs in Pro Tools and Logic that come factory that can compete with the equivalent offered in WAVES, not to mention they ripped off tons of Bomb Factory stuff that I feel Bomb factory did a better job at.
I will vouch for Waves L series limiters being some of the best at pushing loudness hard as hell while staying fairly transparent.
UAD, in my personal opinion get it. I've never heard a UAD plug that wasn't perfected for the consumer it was designed for. They stick to their guns and do an amazing job at reproducing the sound of hardware counterparts at a fraction of the price.
Again, all just my opinion.