I've had a chance to test most of the SPG properties in Delhi the past couple months.
On the whole, Le Meridien takes it for me. The best Platinum recognition of any SPG property in India in my experience -- I have never not been given a suite (with panoramic views of Rajpath and India Gate), in-room check-in, a snack spread (nuts and chocolates), and a welcome bottle of wine. Central location for tourists (or if your business is in Delhi proper). Also a 5 minute walk to Central Secretariat metro.
The ITC Mayura is the most beautiful property, and the most "Indian" vibe. It's also the most accessible from the airport, although you will need a taxi to go anywhere and the traffic at Dhaula Kuan can be horrific at rush hour. Platinum recognition on my one (weekend) stay was negligible, although the lounge breakfast is one of the best I have had anywhere in the world (full menu).
The Sheraton in Saket is underwhelming. Tired lobby and rooms, and zero Platinum recognition. I was asked to show my Platinum card (does anyone carry that thing?) until I came around the desk and showed the receptionist the "P" next to my member number on his computer terminal. A five minutes discussion on free internet benefits ensued. Weak lounge breakfast, as well.
The Westin Gurgaon is brand new, and feels like any other new, high-end Westin. Beautiful lounge, though the view of NH-8 is nothing to write home about. Good access to the airport (besides toll delays), and great access to most of Gurgaon.