first movie - still gets me because nostalgia. also has that 90's-anime art. I honestly cannot say if it is good or bad, because I'm just too damn biased.
the power of one - bad guy was lame, and it bugged me how at the end he picks up his ancient Mew card in the ruins of his fortress and says "…and so it begins again..." as if he didn't learn his goddamn lesson. blah. at least Team Rocket was relevant.
spell of the unown - entei is badass, charizard is badass, unown are freaky, fights are cool. one of the best ones.
celebi 4ever - celebi is such a worthless piece of shit. all it can do is get captured, turn into a wheaties monster, and then fail to get revived. suicune didn't do jack shit, either.
heroes - kind of a step down, power-scale wise. the fossil pokemon were good, but the rest of the movie is just ash dinking around with latias and latios and pointless stuff like that.
jirachi wishmaker - I hate max. this movie is all about max being a whiny baby.
destiny deoxys - finally another good one. deoxys vs rayquaza is fun times, and the city is pretty cool.
lucario and mystery of mew - like Robby said, an actual story to tell. Lucario has some drama and badass moments, Mew acts a fool, the Regis make an appearance, also there's some super-spy chick with crazy hair.
pokemon ranger and the temple of the sea - they just changed the voice actors, so no one sounds right. the villain is ridiculous. Jackie is obnoxious. Manaphy is saccharine and annoying. Ash holds his breath for a solid minute, and also gets to go Super Saiyan for no reason at the end. horrible.
rise of darkrai - mediocre at best. Dialga and Palkia have the world's worst kaiju battle, and even though Darkrai is kina cool, there is entirely too much time spent on some asshole who gets turned into a Lickilicky for some reason.
giratina and the sky warrior - Shaymin is a fucking cunt and I hate it. "thank shaymin time", my ass. Giratina ends up being a joke, and Dialga ends up being involved again. Regigigas stopping the glacier was great, but I wish it got to do more than just that. reverse world was also underplayed. basically, a lot of potential, but is doesn't do anything with it. also Shaymin.
that's all I've seen. If you haven't seen any of them, and are now slightly interested in watching any, I'd recommend the 1st, 3rd, Deoxys, and Lucario.