In my completely personal opinion there is no "best" Weapon Model Pack.
However if you are working with FPSC x10 then MP 9&10 (made by ErrantAI) are the best picks because they are more detailed than the MP 5&6
You can also check out the store, for the price of one pack you could probably also find your personal selection of weapons (by several developers, but most are made by ErrantAI). Especially for close combat weapons you'll find a much broader selection in the store than in ANY Model Pack. You also get to buy a (quiet cheap) Rocket Launcher there, while no MP so far includes one.
if you buy in the store you should avoid buying anything thats allready included in a Model Pack so you'll get no doubles in case you want one of those too.
There are also several stock weapons rigged to the MP6 Hands freely available for download in the forums, in case you want to start out without investing too much:
http://forum.thegamecreators.com/?m=forum_view&t=121596&b=24
http://forum.thegamecreators.com/?m=forum_view&t=156828&b=24
Hope that helped a bit
In case you find my grammar and spelling weird ---> native German speaker ^^
