Suppression of Enemy Air Defence means just that: removing the enemy ability to damage your air units. Not to be confused with Air Superiority, SEAD focuses on destruction of land-based, fixed or mobile air defenses. SEAD is preparation for a follow-up air strike that takes out a primary target, such as an ACU or a Strategic Missile Launcher. This is easily one of the most difficult, yet rewarding, actions to accomplish in Supreme Commander, as it requires a good knowledge of your own units, enemy units and structures, and careful timing.
First thing to do is an accurate reconnaissance. It is vital to know exactly where crucial air defenses are located: shield generators, T3 Anti Air turrets, T2 and T1 AA, mobile AA in that order. It is not necessary, usually, to remove every anti-air defense in the area, just enough to make the follow-up air strike effective.
Then, it is time to assemble the SEAD strike force: the very best units for the job are T3 Strategic Bombers, followed by T2 Gunships and then T1 Bombers. (If UEF, then use the UEF T3 Ghetto Gunship) Alternatively, if you aren't a big risk taker, your SEAD strike can consist of mainly land units to take minimal casualties.
It is important to assemble the strike force in formations about halfway between the starting base and the target: a close bomber formation is much more likely to pass through the inevitable first anti-air volley and do enough damage.
Speaking of first impact with enemy anti-air, it would be better, if possible, to minimize its effectiveness: AA T1 and T2 turrets have poor tracking, so it's important to NOT charge at them directly. Lay out waypoints to attack from several directions at once will keep turrets from getting a good target (See Point defenses for more detail).
A good trick is to use a mass of T3 Air Superiority Fighters to draw fire away from the bombers, by flying them over enemy turrets at a 90° angle from the turret itself, as to present the fastest, hardest to track target.
UEF Broadswords and Experimentals however mostly do not need diversion tactics like this one: they're so heavily armored they can easily sustain a concentrated fire enough to destroy their target.
Again, it is very important that a successful SEAD strike be followed by the main bombing squadron as soon as possible.
Apart from unique units, the best Strategic Bombers for the job seem to be the UEF Ambassador, being more heavily armored than other T3 bombers, powerful, and with an added gun for anti-air protection, and especially the Cybran Revenant stealth bomber, which makes up for having less firepower and armor by being able to become untargettable by radar until it gets on top of its target. Aeon Shocker bomber, even if slightly more powerful than the UEF, is neither as armored nor stealthy, giving it a hard time in actually reaching its target before being destroyed.