Best to alch is dragon halberd alchs for 195 k each, can only get 10 at a time though. So not good for long alching, next best is air battlestaffs if crafted you can make some good profit if bought loss is very minimal. Battlestaffs you can only get 100 every 4 hours though to so.
http://runescape.hkgame.de/cheap_alch.php check this page - best items for alch edit: oww some1 posted that yet
You can go to the lost city of Zanaris, in there there is a Gem shop. You can bring tons of nats, and 30k cash. You buy Ruby Rings from the store, then alch them. I did this when nats were 235 or so each, they've gone down lots since then. At the time, depending on how much I paid for nats, I lost 15-30 Gp. With the current price of nats, you will lose 0-15 Gp depending on the price you pay for nats.