This is hard, this game of identifying the northernmost, southernmost, westernmost, and easternmost points of San Francisco.
If you guess, you’re going to make mistakes.
All right, in no particular order, here are the answers:
Seal Rock, aka Saddle Rock
Red Rock
North Farallon Islands
Alameda Island
I’m srsly.
1. Northermost: Red Rock Island. It’s about eight miles north of Fisherman’s Wharf. Seems as if it’s always for sale. You can get there by kayak and then hike to the top if you want – put that on your bucket list.
As seen from the mainland of San Francisco. The Richmond San Rafael Bridge and East Brother Island are in the background:

Red Rock at 600mm on a very clear day. Don’t try this with your iPhone camera – your shot might not come out so hot.
2. Southernmost: Seal Rock, South Farallon Islands. It’s south of Southeast Farallon Island. It can be seen on the left side here.
And here it is as seen from Twin Peaks:

840mm, on a very clear day.
3. Westernmost: North Farallon Islands.
Actually, it’s this islet in the upper left here, seen with North Farallon Island and St. James Island in the lower right. (I’m surprised that this unnamed rock hasn’t yet been named for a longtime local political strongman like Willie Brown or Rose Pak):

4, Easternmost: Alameda Island. Surprise! Work out your denial issues here.
As seen from the (old) Bay Bridge, the eastern span, the one that was never named for a local political strongman, like Willie Brown or Rose Pak:

Here are the four points, underlined, to sum up:

Oh, even better is if you let Google Maps highlight the county line in red. (If you zoom in on the linked page, you can also see a couple of places on Angel Island that Google similarly correctly ID’ed as parts of San Francisco.)

Thanks for playing!
UPDATE: Oh, man, you people are clever:
1. So what about SFO airport as southernmost? Well, that place is “owned and policed by the City and County of San Francisco, but is located in and entirely surrounded by adjacent San Mateo County.” So that’s why the San Mateo County Coroner was involved in the SFFD’s less-than-perfect handling of the Asiana 214 accident. Another situation would be with San Francisco’s O’Shaughnessy Dam up in Hetch Hetchy near Yosemite – a murder on top of that dam would be prosecuted by the Tuolumne County DA (or maybe the Feds – you all might be right, IDK), not by the SFDA. Don’t ask me about which DA would handle a stabbing at San Francisco Jails #5 or #6 down in San Bruno, San Mateo County. (Perhaps the counties have an agreement over such matters? IDK.) OTOH, a crime on the western tip of Alameda Island would be handled by the SFDA.
2. And Candlestick Point (and the entire southern border of mainland SF)? It’s ever just ever north of the southernmost part of the Farallones – that’s a real close call.