I will probably be burnt at the stake for daring to mention this on the Master Race forums, but I loved Halo: Combat Evolved and still love it today. That's the original one (and I love the HD remake Anniversary one, superb job on that one). I really liked Halo 2, but not as much as the first (but it was fun for the most part). I never completed Halo 3 (but played it quite a bit), barely played ODST (but saw a good portion of it), played a lot of Halo: Reach (and saw its ending but didn't get there myself, I was just seeing my cousin's game progression one day and he was completing it so I kept watching, the game was pretty good overall I'd say) and I never played Halo 4 so far (but saw lots of footage and video reviews of it, and some spoilers of it too but I don't mind much about that). That's because I owned the original XBOX console, but then I skipped the last generation of consoles (PS3 and 360, although I do own a Wii but I bought it very late and I only own two games for it, but that's another story) so I never really played much of Halo past Halo 2 really (the others were all rents with my cousin and we played them on his 360 occasionally, and he eventually bought the Anniversary one along with Reach, so we played those two specifically quite often but I never completed them myself).
If I did own at least a 360 I would have most likely bought them all (not saying I would have kept them all though, I don't have that much of memorable stuff to say about Halo 3, it seemed rather generic but I never saw all of it). As far as the first is concerned most of it, for me, was (and still is) memorable. The soundtrack is awesome, very atmospheric at times and very fitting for the game overall, and the series too (that's probably the most memorable part, and you don't need to like the series to at least like that music). The enemies, their sounds and voices (nothing beats the original Grunts voices, and the almost iconic non-recognizable/understandable Elites speech). The weapons (and their sounds too) and the famous (or perhaps infamous, but I liked it) weapons carrying limitations or the regenerating shields (both yours and on the enemy Elites too). The superb difficulty balance and the great, memorable Elite difficulty setting (Halo 2's Elite difficulty wasn't as fun in my opinion due to the sniping Jackal units, they could one-shot you, that includes piercing the shield even if it's full and draining all your health, a literal one-shot, and because of that it made Halo 2's Elite difficulty a pain rather than a fun challenge).
The levels, their layouts and their respective settings as well (won't mention too much here due to potential spoilers), especially the third one (night time setting, with memorable battles mostly the section that leads to the fourth level), perhaps excluding the mid and/or later levels with more repeated environments but honestly they never bothered me that much. Also, the driving parts and the great controls and physics despite the game's age are fun (well I thought it was great, that's very subjective I guess). And perhaps one of the most important (and memorable part for me) feature the game offers being its co-operation mode. I can't recall all the laughs and fun times I had playing on co-op Elite difficulty with my cousin, so many places gave us challenges (and frustrations too), but it was awesome. I could go for quite longer about the first game alone. Maybe one day I'll buy some cheap 360 and get used copies of the whole series once more to finally complete the ones I didn't, and have a better judgement of the entire series (actually I should do that sometime). But I can say for sure that, for me, the first game was the best so far (especially the Anniversary one, it's better than the original, but the original still has its charm and plays well but it will seem dated, they should have also remastered Halo 2 the same way) and the sequel was very fun for the most part (but you need to keep going beyond Halo 2 because its ending will leave a bad taste for a while, so better launch Halo 3 soon after, in my opinion).
Additionally, most of the A.I. in the Halo series is some of the best you'll fight against in comparison to a crap load of other story-driven single-player FPS games, with the exceptions of some enemies here and there which have simpler A.I. patterns due to their actual nature and "personality" rather than because the coders didn't bother to work on it (and that's no exaggeration, just play on Elite difficulty and tell me Elite enemies were easy all the time, they take cover, come out of cover for fast hits-and-runs and move to another spot, they let their shields recharge, they throw grenades, they work in groups, they try to attack from other paths when available to attack you from the back or they try to flank you, etc). And the story, the pace and ways at which it unfolds, and the interaction between the protagonist and the A.I. Cortana being what "drives" that story's unfolding, all of that was well crafted (but it's nothing mind-blowing story-wise, it's nothing that complex, I'm simply saying that it is well crafted and told to the player in a smooth way and all levels transition well from one to the next). It flows smoothly and I never felt "lost" in the story at any point, there's of course some lack of details because at the time the trilogy had not been completely written and they pretty much went improvisation-style (I.E. writing as time goes on rather than planning the whole trilogy or beyond the third title from the start) but overall the story and what's actually happening is "understandable" and, for me, it was enjoyable.
To be honest I never understood the "hate" that some people have against the Halo series (and I do refer to actual "haters" per se, not the gamers who actually have valid complaints or disappointments while at the same time can see that there's at least "decent" or even "fun" parts or game-play mechanics here and there). I think that some of that hate might come from the fact that for some reasons the brand symbolized Microsoft's success (and its console) in the video games market (at least at the time anyway) and because people loved (and still do today) to hate Microsoft they also had to hate their video games branch "baby". That or perhaps it had something to do with a frustration of non-XBOX owners at the time that only Microsoft's console had "the best FPS game" of the consoles trio and - again - the gamers refused to buy the XBOX "just for it" and if not, it was "because Microsoft"). And I'm referring to the original game's success here during its respective time. It had critical success (regardless of the haters) and it lead to a legacy, it's a saga at this point and I'm sure we might get a full feature film of it one day. Maybe today if there's any of that hate remaining people will point at actual game-play mechanics rather than just going berserk on it, who knows.
I'd still give a solid 9/10 to the original (Anniversary version especially, but the original too) and a good 7.5/10 to the sequel (very fun for the most part but I had mixed feelings starting from a specific point which I won't mention due to possible spoilers). I played a lot of Reach and saw its ending and most of its important story-related scenes and I think I could say that it might be the best of the series since the original. As far as Halo 3 is concerned, from what I've played I'd say maybe 7/10 or so (maybe I'd like it more if I played it more, or maybe it'd be the opposite). I'd say that Halo ODST was a fresh and interesting point of view (you play as a different character) but I haven't played nor seen enough of it to judge it properly. And, concerning Halo 4... can't say anything yet really other than it seems interesting and I know the dev team is completely different but is a canon sequel to Halo 3 (from what I've read of it).
Yeah ok, also, here "TL;DR" version of this wall of text being "I actually kinda like the Halo series".