It depends on the specific major - Business Economics or Financial Economics, as they have different plan structures.
For Business Economics (what I'm doing): it isn't so much a case of which are the most easy/efficient/interesting, but moreso deciding which will meet the prerequisites for the 2+ List B courses you'll need to choose. My advice would be to look through the List B courses, decide which are the most interesting to you and then work backwards to ensure you meet all the prerequisites.
Here are some level 2 courses (from List A) that I've done and found easy/interesting:
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ECON2102 Macroeconomics 2: although not compulsory or as popular as Micro 2, I found it worthwhile and an easy WAM booster
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ECON2111 Globalisation: micro and macro theory on trade and financial crises, straight from the prescribed textbook.
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ECON2104 Applied Macroeconomics: a research-y subject consisting of 2 essays and a final (made of up of short-answer questions). Tonnes of great topics and you're given the choice of topics to research
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ECON1301 Australia in the Global Economy: easy level-1 research-y course. 2 essays and a final; the questions asked in the final are the same as the essay topics, word-for-word
NB: My list B courses are
ECON3104 International Macroeconomics and
ECON3116 International Trade Theory and Policy, so the only prereqs I really need are Micro 2 and Macro 2.