You should explain why POT is horrible.
You'd see it from the very first episode. PoT, at least early on, is about nothing more than some snot nosed kid beating the crap out of older kids at tennis match after match. I seriously don't understand the appeal in that. In fact, PoT is the ONLY sports anime where most people end up rooting against the main character because of how much of a snot nose brat he is.
However, the series' author apparently realized how stupid his manga was, and eventually the show starts doing episodes that aren't related to tennis, where all of the characters just go out and do really crazy and stupid things. I haven't seen these episodes nor have I read the manga that far in advance, i'm just taking the word of a fangirl for it. But is it worth it to drudge through all of the crap to get to that? I think you should just watch a good sports anime instead, like Yawara.
oh and here are my favorite long anime series:
Ghost in the Shell Stand Alone Complex (52 episodes) - only if you like cyberpunk/sci-fi/science babble
Himechan no Ribbon (61 eps) - Possibly one of the best magical girl anime, certainly one of the few that are watchable for more than an episode or two.
Hana Yori Dango (51 eps) - Great, if you dig shoujo high school drama, consistent quality in each episode up til the very end
Also, I enjoyed Tenshi na Konamaiki and Mujin Wakusei Survive, but haven't seen the series to their completion (only about half for each of them). They are both around 4 seasons (or 48-52 eps).