Leicester Square is generally a very poor place to try to get a drink. Pete is right that De Hems is your best bet in this area. Further options range between the limited number of other OK-but-very-full pubs or the numerous unspeakable-but-full pubs. Particular offenders are the chain pubs in the vicinity (ABO, Wetherspoons, Hogshead), which offer both a crowdy, noisy and smoky drinking environment and offer you the chance to buy THE THREE POUND PINT.
The Pumpkin Pubs Cnut-like campaign against THE THREE POUND PINT starts here.