wich lore you will want to use most of the time will depend on your army build and playstyle. Life is always a good choice and is even better if you like to use multi wound units. light works very well with large saurus blocks, and shadow or death are good for a slann without temple guard. my suggestion would be life simply becuase a slann painted for life will not look strange even if you give him another lore.
Life's supposed to be the most competetive, and all the armies you listed are T3, so dwellers will be good against them. Light's also good against undead as it does extra damage. Death is great against skaven as they rely on their characters for Ld if you can snipe them you're halfway there. Come to think of it TK rely a lot on characters too. But overall Life is generally the best all-round lore for lizardmen. That's 2 votes for Life. My slann is painted normal (green) but he isn't glued down and I am considering trying to pick up some other slaan so I can paint them differently and change them round depending on the lore.