Just to add to the divisiveness in this thread, Reach is probably the best Halo game. I haven't played Anniversary though, and I'll admit Halo 1 was pretty special but it's aged a little.
Halo 3 is utter shit.
No. That's just plain wrong.
Reach is amazing and by far the easiest to recommend to someone who has never got into Halo before.
Halo 1/Anniversary - looks/sounds great, some awesome design - but hasn't aged all that well gameplay wise. Repetition and bland weapon selection have driven away quite a few people that I know IRL. Probably my favourite campaign.
Halo 2 - A mixed bag at best. Campaign was ambitious and creative for better or worse - but again, hasn't aged all that well. On top of that, the visuals aren't great so it would be difficult to recommend to someone who isn't in love with Halo.
Halo 3 - This game is amazing for anyone who loves the expected Halo gameplay. I loved the game - probably my most played campaign/co-op wise. It is not in anyway, utter shit. The battles and variety in encounters are easily the best in the franchise. The weapon selection is fantastic, and it's just very well thought out and designed in general. The issue here -is that from an audio/visual standpoint, it's subpar. I think some elements of it are brilliant - probably the most impressive water I've seen in a game, or at least a shooter - the lighting effects are great - and the scope of the levels/encounters leads me to believe there wasn't much wastage in terms of the Xbox 360 horsepower available to Bungie.
However, compared to nearly any shooter that has come out since - it looks and feels totally subpar. The visuals are a mixed bag at the best of times, and a total joke at worst (the Plasma pistol looks comical in 2012) - and the guns/shooting/explosions etc. are all muted and boring. There is no element of excitement walking into a new area, or picking up/firing a new gun - because the whole game feels flat from that perspective.
Despite it being a fantastic game to play - there is very little reason to recommend it to anyone. Well, that's not entirely true - as a Halo fan, I want to recommend it to everyone. However, if you wanted people to see Halo as many of 'us fans' see it - you would start them at Reach, or even Halo 4 in November... because Halo 1/2 have aged enough that I don't imagine (and certainly from my experience watching people play) it's all that fun to persevere with the aging mechanics until it 'clicks'. Worse still, Halo 3 - despite being an old game by many standards, is still the last mainline entry in the series. It has the unfortunate position of being borderline ugly at this stage in the console lifecycle. More importantly, anyone who was to forgive the visuals and play it because they wanted to see what the game was like before say, Halo 4 arrives - would have to endure extremely basic audio/visual feedback during battles (obviously a massive portion of the game) - which wouldn't be an issue if there wasn't leagues of COD/Battlefield/Killzone-type shooters that absolutely nail the 'guns are fun' feel.
I went a bit all over the place there - but I just wanted to state again WHY:
A) Some people think Halo 3 (or even in some cases, Halo in general) is shit
and
B) That it's not a game to recommend to franchise newcomers
I wish Halo 3 was still pretty by today's standards (I thought it was gorgeous at release, and I'm still quite fond of the visuals) - but I think Halo Reach captured a best of both worlds that should represent what Halo is about. Big scope, great visuals (Reach looked fantastic despite the ghosting), acceptable-if-not-great gunplay, and a well designed campaign.
tl;dr - Halo Reach is best Halo, if you are a franchise newcomer.