tisk, tisk, tisk you should never have someone tell u what army to play my friend. It also personal taste, each army has it's own unique style troops, an army can beat any other army when played right (note that some armies may be harder to win against then others though) Here is what i see as the strengths and weakness of all armies
Eldar- Strengths: lots of units to choose from, best transport, psychic powers,
weakness- standard weapon 12" range, low strength and toughness
DE- im sorry i just have no idea here never played with or against them
Orks_ Strengths: higher toughness, fun to play with, lots of troops (choices and actual models). looting!good in Hth
Weakness: Bad BS, Bad armour save, lots of mini's to paint
Guard: - Strengths Tanks, ordance blasts (note thats plural) lots of massed fire power
Weakness: sucky in HtH, need an entire platoon to warm a soup can with their lasguns on maxium fire rate. Lots of mini to paint
Space Marine (this is generic, for all chapters): most platic models of any race, high stats straight 4s almost, variety of wargear and weapons, taticaly forgiving
Weakness, higher in points for models, over played, most army list in tournaments are anti-marine in design.
Tau: Strengths: fire power, lots of deep striking units,
weakness: - horribly in Hth, bad BS (for what it should be),
Necrons: Strenths- auto glance on 6s, good armour save, C-tans, monliths,
weaknesses- 75% phase out rule, somewhat lacking in HtH,
Chaos Strengths: generally same as marines, plus all those cool chaos troops
Weakness: no and they shall have no fear rule, same as marines in generaly
these are totally my own opinion as from experience, hopes this helps you out