I was thinking a lot about this thread's topic, because I was having a lot of trouble thinking about anime that I regularly "go back" to – I usually watch a show all in one go – but, then, I realized there was an obvious answer right under my nose.
Yat-TA Yat-TA Yat-TA-MAN!!! (Yatter-WAN! AHAHAHAHAHAHA!!!!!)
The entire Time Bokan franchise fucking rules, but
Yatterman is like a series specifically engineered for my tastes. The artwork and character design is phenomenal, all the characters are truly hilarious (I always like it when an anime series can make me literally LOL), and the formula that the show is built around is absolutely fantastic. This series and its characters has inspired a LOT of Japanese entertainment, and it's very easy to see why – it's impossible not to love both the heroes and the villains, and not to laugh at their insane, outrageous exploits, even as a cynical adult. The writers of every Time Bokan show know that parents are watching, too, and include a load of outrageous jokes and dialogue that regularly have me howling.
Both the original 1979 show and the 2008 series are excellent, and I'd say that each has their own advantages (Ai's verbal tic in the newer series is the most endearing thing imaginable). I really like how Yatterman aims for a very wide audience, and hits it all – kids enjoy the heroes, adults enjoy the villains, crazy internet people enjoy jacking off to the villainesses, and twentysomethings who still watch cartoons enjoy a very clear sense of effort and engagement from the creators. This dumb children's robot franchise really makes me happy –
da koron!
Also, I think that, in terms of objective quality,
The Tatami Galaxy is the best anime series I've ever seen (not necessarily my favourite, but it's still up there). I think about this show a lot – the music is phenomenal, the character design is quite literally perfect (in that I can't think how I would improve it, I love the way these weirdos look), the writing is fantastic, and just being in this world is like going to another universe of colour and imagination. Masaaki Yuasa, I hate to say it, is a complete genius, and probably one of the best animators working today.
The scene that's always stuck with me about The Tatami Galaxy is when the main guy is in a drinking contest against some big dumb jock to win over a hot older chick. The way that the characters' bodies gorge and distort as they get progressively more drunk is a Ren & Stimpy-style portrayal of emotion that I didn't even think was possible until I saw it. It's a bunch of arty hipster bullshit for people who hate everything, but it's also a really great show.