What kind of property do you like?
A moderately priced 4 star that's very reasonably priced is the Le Meridien (formerly Park Hyatt). My new favorite top hotel in San Francisco is the St Regis.
Neither of these properties is located right at Union Square which has the biggest concentration of "touristy" things to do - shopping, restaurants, cable car lines etc. But bear in mind that San Francisco is actually very compact overall so unless you're talking about going out to the Golden Gate Bridge area, its easily walkable or use public transit (I say this even though I rent a car every time I'm in SF). Le Meridien is probably about 7 blocks from Union Sqaure but only about 3 blocks from Ferry Building (farmer's market on weekends, building has upscale food shops, and is also the terminal for ferries to Sausalito / Larkspur etc). St Regis is probably 3 blocks from Union Square and 7 blocks from Ferry Building.
You're probably way too early to finalize hotel plans for next May. The hotel market in San Francisco is really hurting right now and there are amazingly good deals to be had. Le Meridien regularly shows up on
www.travelzoo.com deals with regular rooms around $130 and smallish Suites for $185.
Friends who like Intercontinental seem to like the Intercontinental San Francisco (a property less than 2 years old) much better than IC Mark Hopkins (which is much more old world historic). The new property's immediate location isn't really that great but you are within 4 blocks of Union Square (flat walk) while the Mark Hopkins is atop Nob Hill (nothing but other hotels other than Grace Cathedral in the immediate area with a steep hill down to Union Square).
Right at Union Square, I probably like the Grand Hyatt the best (note that I'm top tier Hyatt Diamond) but the rooms need to be renovated which they have said will start soon. The Westin St Francis's rooms in the "old" historic part can be tiny.