Any and all. I love them all equally from beginner to master.
Imps are a good way as some bosses will drop clues as well (corrupted gauntlet drops elites. Heckhounds (hard), Crystal keys give a chance at some clues. Barrows is another good way especially the wildy ones and opening the keys. There are relics as Xei said as well as an auto clue sover to save you some time (costs 80 bonds).
Imps from hunter @ puro puro
Dagannoths and battle mages for medium clues
Fishing and woodcutting also give a lot of clues tbh
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