I was asking myself the same question when I started out and decided for MP 5 because it has it all:
- unarmed fists for cc
- 3 melee weapons(!)
- 1 handgun
- 1 MP
- 1 sniper rifle
- 6 automatic rifles including 1 with a grenade launcher
- 1 MG
The only thing missing is a classic shotgun but the pack makes up for this with the inclusion of an SF-looking shotgun and several other weapons also dont look out of place in an SF game.
But of course.....it only took me only several weeks to decide, that I need all the weapon packs 5,6,9 &10 
....and even after building my first games I still couldnt claim that I find one of them is *perfect* as well as I cant say, that I regret buying a single one of them.
If I might sum the advantages of the packs up (personal opinions):
MP5 - most versatile pack (see above), comes also with additional hand textures
MP6 - includes dual wielding weapons (no mod required)AND is optical compatible with MP6 hand-rigged stock weapons. Additional hand textures (only gloves, no sleeves) are available as an add-on
MP9* - includes a beautiful dual-wielded Katana
MP10* - includes the only grenade apart from stock media and 2 community-made (german WW2) stick grenades
*both MP9/MP10 provide the best quality regarding texture, sound, decals and animations, the ammo pick-ups are of extreme quality and even come with single & batch versions. They also include one each of the popular zombie-hands and there is also a HUGE number of additional hand textures as an add-on available and a lot of combatible HQ stuff in the store.
That sounds like I am a total EAi fan-boy....but to be honest I still work mostly with MP5 because I like the slighty grungy textures AND one of the the hand textures matches the wonderfull "Stealth Aiko" texture perfectly - shes my favourite character
But the bottom line is, that in the end its all matter of personal taste
In case you find my grammar and spelling weird ---> native German speaker ^^
