What do you mean by conjuring would be the most dangerous form of magic? If magic is intent based then its all equally dangerous really.

 

To expand on what Portus has said:

Let's consider a few uses of the shield charm:

GOF. Can barely stop minor attacks.
OotP. Can stop stronger attacks.
OotP. Can stop a piece of magic mid-effect (Portus' example).
HBP. Can stop a spell sent at Harry by Snape, and now reflects it back.
DH. Stops physical things.
DH. Is cast over the entire Great Hall and stops all magic going on.
DH. Flitwick's Protego Totalum cast over all of Hogwarts.

With Flitwick's Protego Totalum, it's not decisive on the "intent debate" because it's not clear what's going on. "Protego Totalum" may be a different spell, or it may be that it's a kind of extension on the shield charm, just like the way we have seen the spell been adapted in accordance with intent in the above examples, but that the intent has to be voiced with a word.

Anyway, what the above examples are designed to show is that a single spell seems to be adaptable to a high degree. I wouldn't be surprised if "Protego" was the only shield charm there is. But this is repeating myself somewhat. As well as the stuff already linked, I'd recommend this thread:

http://forums.darklordpotter.net/showthread.php?t=9652

 

A point that came up with Dwitty in discussion about the Protego Totalum and Protego Horribilis incantations. Would it not make sense if the reason for the incantation is to become a vehicle of your intent. You could cast those brands of protego with the standard incantation, but you would need great levels of focus and very clear intent. So you help focus your mind and intent more clearly by using a slightly different incantation. I know this is similar to what Taure just said, but take it further. It could be that many spells have 'secondary incantations' which can be used to provide slightly changed effects, a non-canon example could be lumos rutilus. A standard light charm which gives red light rather than white. But similarly the same effect could be produced simply by having a strong enough focus on the incantation lumos and desiring to produce red light.