Which is faster?T1,T3,cable/dsl?

DSL (though this depends, can be faster than T1) < T1 < Cable < T3.

Contrary to what user "good" said, schools, particularly universities, do NOT use T-Carrier and instead use OC (Optical Carrier). My university used to use partial OC-3 but I believe they now use partial OC-12.

If you have the money, you should go for T3 as more bandwidth = faster = more optimal for downloads. Your ISP, however, may shape traffic or have other methods of preventing/slowing illegal downloading, if that's what you were thinking, so you should check the below link for a list.

Well, this question is not quite as cut and dry as it used to be. First T lines and cable/dsl are completely different beasts. You also need to take into account a few variables. Really it depends on what you are doing with them.

I'll start with Cable/DSL.
These are more consumer oriented services. They are asynchronous internet connections - what this means is that your download speed is not the same as your upload speed. for cable I have seen speeds in the ranges of:
3 Mbps down 512 Kbps up all the way to 20 Mbps down 1 Mbps up.
With dsl i've seen:
768 Kbps down 128 Kbps up
all the way to 15 Mbps down 5 Mbps up

If you are doing alot of uploading while you are downloading (with things like bittorrent) you will see a slower download speed because the ACK packets have a harder time getting out to get the next packet from the server.

These tend to be much cheaper, with no SLAs and are more oriented toward consumers.

T lines T1,T3 are synchronous lines which means that they can upload just as fast as they can download. These are normally used in business settings where upload is just as important as download due to hosting your own mail, websites, vpn, etc.

T lines also have an SLC and are generally more reliable. But at the same time Vastly more expensive. A T1 is about 400$/mo T3 lines can run you from 5-8K/mo

T1 lines are 1.5Mbps each, and T3 lines are 43Mbps. A T3 is 28 T1s bundled together.

You can also Multilink up to 4 T1 lines for a max of 6Mbps.

so when comparing them all depending on what is available in your area it would be

Cable or DSL then T1 then T3

DSL (though this depends, can be faster than T1) < T1 < Cable < T3.

Contrary to what user "good" said, schools, particularly universities, do NOT use T-Carrier and instead use OC (Optical Carrier). My university used to use partial OC-3 but I believe they now use partial OC-12.

If you have the money, you should go for T3 as more bandwidth = faster = more optimal for downloads. Your ISP, however, may shape traffic or have other methods of preventing/slowing illegal downloading, if that's what you were thinking, so you should check the below link for a list.

T3 is fastest at 44,736 Kbs
T1 is 1,544Kbs
Cable is up to 1,000 Kbs

IDSN is 144 Kbps
I perfer IDSN it's cheap

but many people don't buy land lines any more opting for cable instead.

ALL T's
they are used for big networks such as school, companies