Recommended Canonical Viewing Order:
Stargate movie
Stargate SG-1, episodes 1.1 to 8.2            (SG-1 seasons 1-7)
Stargate Atlantis, episodes 1.1 to 1.15       (Atlantis season 1, part 1)
Stargate SG-1, episodes 8.3 to 8.20           (SG-1 season 8)
Stargate Atlantis, episodes 1.16 to 2.1       (Atlantis season 1, part 2)
Stargate SG-1, episodes 9.1 to 10.2           (SG-1 season 9)
Stargate Atlantis, episodes 2.2 to 3.4        (Atlantis season 2)
Stargate SG-1, episodes 10.3 to 10.12         (SG-1 season 10, part 1)
Stargate Atlantis, episodes 3.5 to 3.19       (Atlantis season 3)
Stargate SG-1, episodes 10.13 to 10.20        (SG-1 season 10, part 2)
Stargate: The Ark of Truth
Stargate Atlantis, episodes 3.20 to 5.1       (Atlantis season 4)
Stargate: Continuum
Stargate Atlantis, episodes 5.2 to 5.20       (Atlantis season 5)
Stargate Universe

Prerequisites:
Stargate 8.2 must happen before Atlantis 1.1.
Atlantis 1.15 must happen before SG-1 8.13.
SG-1 8.20 must happen before Atlantis 1.16.
Stargate 9.1 must happen before Atlantis 2.2.
Stargate 9.17 must happen before Atlantis 3.2.
Atlantis 3.2 must happen before Stargate 10.3.
Atlantis 3.8 must happen before SG-1 10.13.
Atlantis 3.10 must happen before SG-1 10.14.
The Ark of Truth must happen before Atlantis 4.3.
Atlantis 2.2 must happen before SG-1 10.4

Sequences:
SG-1: 1.1-8.2, 8.16-8.20, 9.1-9.4, 9.10-9.11, 9.20-10.1, 10.10-10.11.
Atlantis: 1.16-2.1, 2.7-2.8, 2.10-2.11, 2.20-3.2, 3.5-3.6, 3.10-3.11, 3.20-4.2.