Halo 3 would be a great place to start... even though graphics wise they are not as good and up to date as Halo reach, this game is just as fun. The forge game may not be as good as you are limited to barely being able to build anything, but online matchmaking is just as fun and there are a lot of maps to play through .
My rating:
Gameplay - 4/5
Graphics - 3/5
Fun - 4/5
Halo 3: ODST, now this game wasn't exactly the best of the Halo's but in a good light it did have some fun campaign missions. it could get a bit repetitive in the night missions but there was some pretty epic and fun momements during the day missions. Firefight was good as always in all the games and in the multiplayer disc there were some cool new maps for matchmaking and forge.
My rating:
Gameplay - 2/5
Graphics - 4/5
Fun - 3/5
Halo wars was a completely new gameplay style to all the rest of the Halo games, to start it was made by different companies. But just because of the difference in Game style, doesnt mean it has to be bad. This game took Halo into a whole new light of strategy army control gameplay. with both sides playable (covenant + humans) you could control a wide variety of units including Warthogs, Ghosts, Scorpions and even the massive Scarabs. etc. All in all i enjoyed this game and if you like base building, army controling, strategy games then this is the one for you.
My rating:
Gameplay - 4/5
Graphics - 3/5
Fun - 3/5
! All of the following things said above is in my opinion !