2 votes for Imperial Guard:
This is probably unsurprising as it is actually quite hard to make a well balanced force using guardsmen, especially if you don't know what you are facing. A tip for guardsmen players out there that I found works well, is move your infantry and tanks at the same time when attacking the enemy as this means either they shoot up your big gun or the lord commissar hidden in your squad of thirty who's going to beat up a unit in the assault phase.
1 vote for orks:
This in my opinion is quite often a fun army to go against as unless they take a load of hard boys, you can always end up killing a load of greenskins, so even a loss achieves a sort of moral victory.
1 vote for Tau:
Well, against tau there's nothing much to say really, if you have some fast attacking troops and a railgun shot or two misses then you're in luck.
The other armies are in my opinion quite tough (i.e. Space Marines) or rare like Daemonhunters. So thanks for reading and please don't be shy to comment and express you're own experiences.