Wooden: Well its kind of hard not to be good looking, provided you have new paint. But any Out and Back, Cyclone, or GCI decked au natural or in the white (with red and blue highlights) are my favorites.
Steel:MF (with some new paint,) LNM, B:TR, Alpengeist, Magnum (especially when viewed from Soak City's Torrent River,) pretty much any Hershey Coaster, and B:KF (haven't seen it since then so thats its name to me)
And as for what makes them look good in my eye its a combination of paint (both how new and the color scheme,) location (especially if its by a lake or in uneven terrain,) the sounds I associate with it, and lets face it, ride/park experience.
A lot of wood coasters are beautiful. There are simple ones like Cedar Point's Blue Streak and Kings Island's Racer and complex, twisted ones like Rampage and anything from GCII.
I think B&M makes beautiful rides, although a lot of them depend on the right colors. Talon looks incredible when the sun hits it and I love the maroon & creme colors on Georgia Scorcher. Schwarzkopf also built a lot of coasters that are visually stunning (not to mention a hell of a lot of fun to ride.)
Mean Streak is the most beautiful wooden coaster to look at...too bad it doesn't ride that way. For steel coasters, my vote goes to Nitro at Great America....the colors and the massive supports through the trees are breathtaking.
When you are on the Wicked Twister midway and you look over Hotel Breakers, Magnum looks good. As you walk through Frontier Town, Millennium Force though the trees looks good. When you first enter the parking lot Shivering Timbers looks good. Wolverine looks good from the lake and from the train. As you go through the train, you past Mantis and it looks awesome, the finish and it looks very intense from the train.