King Arthur's sword Excalibur was thrown into which lake?

Coventina's Well is said to be one such place.

http://tehomet.net/coventina/word.php

A Doctor Who episode had it as Lake Vortigern.

AD 793
The Doctor, in an incarnation after his 7th, is the wizard Merlin in an alternate universe where reality is closer to Arthurian myth. At the end of a series of conflicts, King Arthur's nemesis, Morgaine Le Fey is finally {victorious;} Arthur is killed. The Doctor puts Arthur's body and his sword Excalibur in a spaceship which he places at the bottom of Lake Vortigern in his home universe. Morgaine apparently seals the Doctor in the Ice Caves and becomes ruler of her solar system.

http://www.angelfire.com/scifi/drwho/modern1.htm

Other claimants to be Excalibur's Lake are as follows:

1) Bosherton Fishponds
Lakes in West Wales which claim Excalibur to be within their depths.

2) Dozmary Pool
Most Famous last resting place of King Arthur's sword on bleak Bodmin Moor.

3) L'Etang de Comper
Legends of the Lady of the Lake seem to centre on the Forest of Paimpont in Brittany.

4) Llyn Llydaw
Camlann was fought in the shadow of Snowdon. Bedivere threw Excalibur into the waters here.

5) Looe Pool
Dozmary is not the only place in Cornwall to claim Excalibur for itsself.

6) Pomparles Bridge
Bridge near Glastonbury where Bedivere returned Excalibur to the depths.

http://www.earlybritishkingdoms.com/archaeology/pomparles.html