Really tough to say, despite not having watched a huge number of them. I tend to prefer the more realistic stuff (I believe they refer to it as slice of life?) as opposed to the action anime, but then, my mood changes between what I want to watch. So, in no concrete order...

1. Nana - Probably my all-round favourite. It's a bit more adult than most manga and I like that. The characters & dialogue are excellent. Really, really looking forward to the day that the artist gets back into the zone and cracks out the rest of them.
2. Planates - The art style is slightly off-putting and the lead male character isn't a favourite of mine, but the lead female and the surrounding crew were great. I found this really nicely done. I found it really rather superb as a (predominantly) realistic near-future sci-fi story with realistic situations and themes.
3. Attack on Titan - Best action-anime I've seen, probably.
4. Nisekoi / Toradora! - Can't pick which one I prefer, honestly. But for the high school drama kinda stuff, these would be my faves.
5. Samurai Pizza Cats -

I've probably only seen about 15 anime series so far, though, so... yeah.