My favourites SF&F games...
Disgaea (PS2, PSP, DS) - Prince Laharl, heir to the throne of the netherworld, finds out that his position has been usurped, and sets out to regain his throne. Disgaea is a strategy-RPG (might be best described as a role-playing game on a chessboard), with lots of different systems to learn and figure out (which you do eventually get the hang of it). Funny, quick to play, extremely difficult, and extremely long - expect to be playing this for at least a hundred and fifty hours, levelling up characters and items to insane levels.
Sim City 2000 (PC) - When you first look at it, it is a seemingly simple game - starting from scratch, create yourself a city where people will want to live. But it quickly becomes quite a complicated affair, juggling the needs of industry, commerce, residential demands, a budget, and ever-expanding your city at the same time. This is the best Sim City of the lot - the sequels added too many features, and made understanding the mechanics of the game unnecessarily complicated.
Destroy All Humans! and Destroy All Humans! 2 (PS2, XBOX). Cryptosporidium is an alien sent to earth to explore, research, and hopefully destroy quite a few humans, too. It's quite a funny satire, the first game of 1950's and 1950's SF in particular, the second game makes fun of the 1960's. Both games are good, but the first game is better than the second in regards to the story, and the second better than the first in terms of gameplay - more weapons, more diverse missions, and more stuff to do. Short, but well worth your time playing.
EDIT: How could I forget Ratchet and Clank? Probably not the most serious of SF games, but funny, fun, and quite, quite addicting.