Halo:CE had the best single player campaign till now (arguably)
Halo 2 had the best MP till now (arguably)
Halo 3 was OK. Had lots of online options and the single player campaign was solid, but definitely not the best Halo in the series.
Halo ODST had a nice dark storyline. I actually rather enjoyed it.
Halo Reach had a good single player campaign but MP ended up pretty meh in the long run for me.
Halo 4 has arguably the best SP and MP so far. However, I think 343 needs another another 4 titles under their belt before they can be fully judged. Let's not forget who started the series as well. Bungie is still the reason we have Halo today, we only had MS and 343 to thank for keeping the series going. But can you really blame MS for not wanting the series to die? It generates a lot of sales for them.
It's just that simple.
