Quote from Trader_Herry:
If you wish to have speedy execution (but not at all costs) in the above 3 cases, what orders do you use - "marketable" limit order (ie for a buy, send at a price at or above best ask), or market order?
Simplified:
Case 1: Market or limit@.
Case 2: Market but expect possible slippage or use a limit above if a buy to factor in your slippage.
Case 3: Limit only. Best placed above Market for a buy.
There are other order qualifiers you can add, like fill or kill(FOK), All or None(AON), Immediate or cancel(IOC). Also various order types like MArket w/ protection, market to limit...