I'd go with Onc. Twinkle. It's a miniature Oncidium that grow like a weed, and should flower reliably in many different conditions. And you can get a plant in flower for about 15 bucks. Plus, the flowers are fragrant, and come in white, red, yellow, and pink.
Onc. Sharry Baby would be a close number 2, for the same reasons, except it's not a miniature. Flowers will be different combinations of red, white, and yellow, depending on the clone.
Third choice would be about any Brassavola hybrid. They're all fairly reliable growers, but like to dry out a bit more between waterings. Mine hasn't stopped growing since it came to live with me at the beginning of the summer. It's got well-formed flowers, and is very fragrant at night. Flowers come in all colors, sizes, etc. depending on the hybrid.
All three types of plants should be cheap to buy at blooming size, from almost anywhere. Just look around a bit. Hope this helps...