Please define your purpose. =} Below are based on personal preference.
1. Canon. Its one of the best and oldest Camera makers, and is the top 2 together with Nikon. Personally, I feel that Canon makes better compacts. I have a PowerShot A530 and i LOVE it! Nikons i've tried are generally ok but are harder to use. The menus are also not as good and I did not have much fun shooting them. Stepping to dSLRs, i prefer Nikon's, but thats another story for another day. Panasonic is still pretty new with this camera thing and i dun recommend it unless u tried it and love it. Stay away from sony. They have 3 generations of T cameras. The 1st one can't take a clear pic without flash. I always got blur even bracing against a chair. The second added "anti-shake" technology which i've never tried. The third, added "double anti-shake" technology, so i'm guessing that the second dun work very well. Y introduce things that dun work? Cos... Its a Sony. Like.no.other. =P
2. Sony as explained above. I'm sure the T100 will work ok, but i prefer Canons. As for 850IS and 900Ti, they look pretty much the same. Go for the 850IS. Its cheaper and does everything u'll ever need. How much are these things btw? >.<
3. 850IS cos its (most probably) cheaper and does things better. U get a wide angle 24mm eq compared to 37mm in the 900Ti. Unless u want a really long zoom. Megapixels are irrelevant.
4. Shooting speed is defined in FPS, frames per second. how many shots u can take a second. Its irrelevant for a compact. Only important for shooting sports on a dSLR.
5. No. Get the Canon and be happy. Get PowerShot A series if u are more serious about photography. It adds a full manual mode for u to play with.
6. Absolutely no idea here. Sorry..
Final note. If u just want a camera for fun, take pics on vacations or just for kicks, get any camera. It doesn't really matter because its the photographer, not the camera. The camera doesn't make pictures, u do. =}
Yo! Thanks for your recommendation.. I think I'm gonna get 850IS.. Cheaper & Image Stabilizer.. You are right, Sony not really good.. Haha.. Canon is the best!