Gulliver have been around for years, with good reason. The Cotopaxi acclimatisation programs are all well designed and are based out of the delightful PapaGayo hotel near Machachi. The guides are all top notch (especially William and Sergio), with a good knowledge of local wildlife and plants. Although you might find cheaper tour operators around Quito, I strongly believe that it is worth paying a little more for a trustworthy and well-established company, especially for relatively high-risk activities like mountaineering.