There are so many great UAD plug-ins, it's difficult for me to imagine working without them.
Among my favorites (this week anyway) are the Cooper Time Cube, the dBX 160, the 1176, the EMT 140 plate, the Cambridge EQ, and... well, those are my favorites. I have a pair of the Solo cards, and my only regret is that I can't pick up an Octo today.
There are a handful of plug-ins that are available in both UAD and native formats, and since I am a little light on UA DSP power I use the Maag EQ and SPL Transient Designer as native plug-ins today.
I've been using UAD plug-ins since the days when they were sold by Mackie... so I might be biased.