For me it's definitely the Sideshow 1/6 TaunTaun and Hoth Luke (I know, technically two figures, but they are certainly displayed together!). I love those and can't believe it took me so long to get them.
For me Groot wins hands down. Other lower end cost items have to be the Neca figures and other pieces I have gotten are the older Sideshow Universal Monster figures, Tweeterhead Munsters and Weta pieces which are fantastic. It's hard to say but in 1:6 range Groot wins and I'd never sell him. To add second best would be Marty Mcfly he's another I wouldn't sell.
Mine would have to be Hot Toys Groot - My favorite figure of all my HT's. Runner up would be Avenger's Hulk. Still waiting on my AOU Hulk Deluxe to see if it beats out the Avenger's version.