Her Majesty's Inspectorate of Constabulary (HMIC) said the Dyfed-Powys and north Wales forces "required improvement".
South Wales and Gwent police forces were ranked "good".
The reviews are part of HMIC's annual inspection of England and Wales' 43 forces.
Police and crime commissioner Christopher Salmon said: "We have more officers investigating the most serious crimes and my Help Hub has increased support for victims.
"There is always more to do but I am confident that Dyfed-Powys is effective and keeping people safe and reducing crime."
Police and crime commissioner Winston Roddick said: "Over five years North Wales saw a 17.4% cut in crime which is better than the England and Wales average reduction of 12.6%.
"The chief constable accepts, as I do, there is always room for improvement and together we must make sure that the improvements recommended by HMIC are carried out as soon as possible."
Police and crime commissioner Ian Johnston said: "It is pleasing to hear that all of that hard work has been recognised by HMIC and that the force has been found to be effective at protecting from harm those who are most vulnerable.
"This report is very good news and the challenge now is to build further still on the findings of the inspectors."
Alun Michael, police and crime commissioner, said: "This report recognises that the priorities set by the chief constable and myself reflect a strong commitment to prevent crime, to support victims and to work in partnership.
"This is set out in our latest Police and Crime Reduction Plan, published last month, which had a strong focus on early intervention and prompt, positive action as well as effective partnership working."

Part of Ysgol Glan Clwyd in Saint Asaph will be demolished in November, with new classrooms, assembly, music, drama and dining areas created.
Work is aimed at meeting growing demand for Welsh-medium education.
Denbighshire council's cabinet will be asked to approve awarding a contract for the work to Willmott Dixon.

Its new official forecast is for the gap between spending and taxes still to be a deficit of £7bn in 2019-20, compared with the Office for Budget Responsibility's forecast made at the last budget for a surplus of £7bn.
Its gloomier projection may be seen as a particular embarrassment for the Tories, given that unlike Labour and the Liberal Democrats they are committed to generating a surplus on the overall budget.
However a small part of the forecasting difference between the IMF and the OBR is that the IMF assumes there will be a weakish minority government after the election and therefore spending will be a bit higher than the Tories' plan.
So a senior Tory put a brave face on the IMF's disagreement with the OBR and said it showed the risks of voters not giving them a clear mandate.
However that is not the whole story, in that the IMF also believes the OBR is being a bit too optimistic about growth and tax revenues in the latter years of the next parliament.
In truth therefore the IMF is highlighting that whoever leads the next government may be forced to increase taxes or cut spending a bit more than currently planned.
This may represent a bigger headache for the Tories than Labour or the LibDems given that in the last few days the Tories have committed themselves to £6bn of tax cuts and £8bn of increased spending on the NHS, without saying where the money for these giveaways would come from.
In fact just yesterday a senior Tory pointed to the OBR's projected surplus for 2019/20 and said that would cover (almost) the NHS commitment.
Well the IMF today says that £7bn surplus is a chimera.
All that said, the IMF forecast is a million miles from being devastating for any party: the difference between a £7bn deficit and £7bn surplus in 2020 is just 0.6% of GDP or national income - which is a rounding error in the context of a deficit that was a humongous 10% of GDP just five years ago.

The man, in his 30s, was found with chest injuries in the red Mazda in North Way, Barton, just after 22:45 BST on Tuesday. He later died in hospital.
Police said 34-year-old Rafal Stawski of Mayfair Road, Oxford, had been charged with one count of murder.
A 31-year-old man, from Oxford, also arrested in connection with the man's death has been released without charge.
Mr Stawski is due to appear before Oxford Magistrates' Court.
Two of Wales' four police forces have been told they must improve how they keep people safe and reduce crime.
[NEXT_CONCEPT]
A £15.9m Denbighshire school construction project is set to get under way after a firm was chosen to carry out the work.
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The International Monetary Fund has today highlighted the challenge to be faced by the next government in returning the public finances to balance.
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A man has been charged with the murder of a man found fatally injured in parked car in Oxford.