I only played MGS 3 and really liked it. Was definitely one of my favorite games for the system. Didn't get into MGS 2 (played maybe an hour) since I started with the 2 PSP MGS games so wasn't use to the old-school control scheme.
Pretty sure starting from Solid would be the best route.... from what I've read, the earlier games aren't really relevant.
Although the games themselves are fairly self-contained. Although I'm sure I missed out on a lot of stuff, I enjoyed the series starting from whatever the first PSP MGS game was and then playing Peace Walker and MGS3.