Started with beaky marines and orks. I've always had orks.
Drunken, goofy barbarians. Plus these guys are tough and nasty and yet still have this fun vibe going on that I don't think any other army could pull off without being stupid.
Don't Press Dat!, Old squig catapults that would hurl the crew by mistake, exploding dreadnought arms, etc. all make it fun.
When I used to play in gaming clubs, I would get tired of "competitive types" bending the rules, getting genuinely angry at dice rolls and losing. I figured this out, ALL gamers should field an ork army at least one year. It'll really calm them down and make them take the game less seriously.
Also, marines. They are the true heroes of the Imperium. I always go back to them. And even with the hordes of Chaos, I still feel that Orks vs. Space Marines is the classic
40k match up.