I've gone with Alien/ Predator but I can't believe I actually thought about Harry Potter
Here's my thinking (and of course this is all just my 2 cents)
Alien/ Predator: Alien, Aliens and Predator all great sci-fi, the first two alien films in particular for avoiding the sequel being just a remake and really changing the focus from hardly-seen suspense to intergalatic war movie. Predator 2 was suprisingly good considering they didn't have Arnie, nice bit of humour and it didn't hurt the series. Personally I dont think Alien 3 and 4 added much to the series but they were watchable. All AvP had to do was be mediocore and I probably would have loved it but instead it was a mess. Still enough good movies to top the poll for me.
Batman: loved the first movie and the recent restart but
Batman Returns onwards wasn't great.
Harry Potter: One of the most even series, if you liked the first you'll like the series, in fact this series has gotten better, which has got to be unusual. Hardly my favourite movies but I have found them watchable (although I thought they added too much of the real world to the last one) which is a lot more than I would have expected and I can understand how someone more in its target audience would really enjoy them.
Matrix: Isn't a series it's a classic movie but they stopped there.... no dont message me.... They Stopped There... never was sequels... never needed to be
Spiderman: First two were ok though the last one hurt it a bit.
Star Trek: probably one of the most up and down series. Wrath of Khan and First Contact I think are actually decent movies, the rest are hit and miss.
Star Wars: Split them up and I think we have a winner
but combined I just dislike the prequels too much.
Terminator: Love the first two movies but if Spiderman 3 hurt the series a bit then T3 killed it, buried it, dug it up, did unspeakable things and then killed it again.
X-men: Not sure on this one, would probably need to see X3 again but I liked the first two so at the least a good effort that used an esemble cast of superheros well.
Lord of the Rings: I'm not actually a huge fan though I did enjoy them and will probably watch them again at some point but I don't think I could sit through all 3 even in the space of a weekend. It probably is the strongest series out of the lot but the reason I didn't vote for it was that it was scripted/filmed etc pretty much at the same time so it's almost an extended movie that was released in segments which I thought meant it faced less issues then most of the other series.
For ones not on the poll how about Indiana Jones or Back to the Future