Odd One Out

[ez64]

This one is too easy but here...

1. CoD 1
2. CoD 2
3. CoD 4
4. CoD 6 ( Modern Warfail 2)

all had there own engine apart from 4?
all good games apart from 4?
the successor had more features apart from 4?
online play was skill based apart from 4?
all reasonably priced on release apart from 4?
all PC ports or multi platform development apart from 4?