I think the best place to start Mogwai is with Happy Songs With Happy People, it's veru good yet still accesible. Young Team is better, but it's a very long album and it may be harder to get into at first. My favourite thing by them is My Father My King (as Praesarpium Machinarum suggested), but it's only one very long song, which may not be the best investment. Some of Mogwai's mid period albums are more restrained, and some (ot me) find them less interesting.
In my opinion, it should go:
1.- Happy Songs For Happy People
2.- Young Team
3.- Mr Beast
4.- My Father My King
5.- Ten Rapid
6.- EP+6
7.- Rock Action
8.- Come On Die Young