There are many options as far as distance, gain, and trails used. If you want the shortest hike than Slide would start from CR47 which is south of RT 28 and it is a yellow trail and 5 mi round trip. It isn't very exciting. Your better off hiking Wittenberg because the view is incredible and it is a great trail with some easy little rock ledges to climb. You can also get to Slide Mt from the giant ledge trailhead and from the woodland valley trailhead near Phonecia. The best hike in the Catskills is probably Wittenberg, Cornell, and Slide and will take all day and is 14 mi out and back but can be made shorter with two cars.
Hunter has many routes. The shortest would be from 214 south of the town of Hunter/Tannersville and there are two options. You can take the Devils path or a shorter route a mile north of the devils path. They will be around 4-6 miles RT.
My favorite hikes and views in the Catskils are Wittenberg and Giant Ledge. The tower on the top of Hunter is a good view too and the Devil's Path is nice and steep.
Use the internet...it has all the answers. The trail maps are also excellent quality.