i cant remember the names of them... but the uncharted 2 beta MP maps are really good....
Also cod4- Crash is v good

1 : Cliffside ( Cod Waw)

2 : Radec Academy ( Killzone 2)

3 : All Quake 3 Arena maps.

Does it have to be FPS?

deck16, de_dust, 2fort for me :)

CTF Giants (UT) DE_Dust2 (CS) & No Mercy (L4D)

maps I played the most and liked best were on Socom 1 & 2 on PS2 :
Crossroads
Desert Glory
Fish Hook
it's been a while but i still remember every inch of those maps...

Ok, now that I have read the OP, here is my list:

5 - Helms Deep (Warcraft III)

Seriously, this custom map rocks, why can't I find it being played more often? :(

4 - Moscow Russia (Call of Duty 2)

Great game, great map, now if only the game had decent online and bots to fill the map with players.

3 - De_Dust 2 (Counterstrike)

Counterstrike is THE FPS for many, and this is THE map to play on CS. FPS map design at it's finest.

2 - Dailan Plant (Battlefield 2)

Every map in Battlefield 2 with an aircraft carrier is my favourite. Playing as Chinese or MEC being an anti-tank trooper and sneaking up on one of them, positioning oneself on the roof and taking out unsuspecting aircraft before they take off is one of the most fun thing you can do in a FPS, ever. I have said it before, and I say it again, Battlefield-style FPS >>>>> All other kinds of FPS.

1 - Destination (Starcraft)

The most popular professional Starcraft map. Many many good games have been played on Destination. My map of choice in any matchup. Destination makes for fast-paced and often very interresting matches.

gg