ME gets my vote. Compelling and chilling.
I liked how humans got involved with other races. It had alot of backstory too if you read through the codex or journal. How humans first discovered the technology that got them up to speed they had a war with the one guys lol. Very indepth though. And the whole reaper thing they looked like animals one was an alienlike squid its like they gather millions of dna of living things and make a super self (if you've played the 2nd one) So i'm guessing the squid one was very ancient maybe 1 of the first. Its the leader
We will see in the 3rd installment about the reapers hopefully.
I'd say Mass effect for my vote.