Veigar's Event Horrizon is the best. Using SS and just walking right through it is priceless.

No Offense taken. Proper testing is ALWAYS appreciated. That is just what Ive seen in my many games as Sivir.

Good to know about DI (not that its easy to SS or anything) and I havnt been in a game with karthus since the new graphic (been smurfing teaching my new friends, side note: OMG its rediculous what you can do at that level and get away with it. Nice change to the tryhard games I have to play on my main). But Im nearly CERTAIN that was one of the spells I counted as "free mana" along with the other "easy" spells to shield (Dark Matter and Even Horizon being the top 2, nothing better then laning against veigar in mid).

Just figured Id give you a heads up, and some of that, I know 100%, but it will be better for readers if you test it as well (as there will be no doubt at that point).

My personal favorites:

Gangplank's Parrley
Ashe's Enchanted Crystal Arrow
Ashe's Volley. (Pretty much all of Ashe's offensive abilties)
Zilean's Time Bomb (Purely because it requires pattern recognition and being able to outplay your opponent)
Cho'Gath's Feast. (Omnomnom? No, no, no.)

one skill i thot should be mentioned:
does SS block any of the bounces from an enemy sivir's ricochets? (similarly, the bounces from kat's bouncing blades)? my guess would be no, but i think that is something that your guide should have in it.

Updated with Karthus, Nasus and Singed.

Sadly, wall is NOT blocked. Once I get around to filling the rest of the blanks, I'm going to post a bigger bug report...

Rammus Powerball is blockable. Not sure if it hits minions and gets you as a secondary, but I blocked a headon from it. Took him out of powerball, too.