In a BBC interview Palestinian PM Shtayyeh said that the presence of Arab countries' ambassadors when Trump announced his peace plan doesn't necessarily mean those countries endorse Trump's plan.
So [entertaining this viewpoint], which Arab countries have expressed unambiguous, explicit support for Trump's plan?
None.
Looking through Wikipedia's article on the plan and Al Jazeera's summary, the reactions can be summarized as:
Additionally, the Arab League has unanimously rejected the peace plan.
Given the complexity and sensitivity of the situation, unambiguous support for any first-proposal agreement would be highly unlikely.
The following states from the region offered support for the effort to open negotiations.
Saudi Arabia
Saudi Arabia's King Salman reassured the Kingdom's commitment to the Palestinian issue and Palestinian rights, in a phone call with Palestinian President Mahmoud Abbas, the Saudi state news agency reported early on Wednesday.
The foreign ministry said in a statement that it "appreciates the efforts of President Trump's administration to develop a comprehensive peace plan" and also called for the start of "direct peace negotiation between the Palestinian and Israeli sides".
Egypt
Egypt urged Israelis and Palestinians to "carefully study" the proposal. The foreign ministry said in a statement that the plan favours a solution that restores all the "legitimate rights" of the Palestinian people through establishing an “independent and sovereign state on the occupied Palestinian territories".
Egypt, which along with Jordan is the only Arab country to have made peace with Israel, said it appreciates the US administration's efforts to try to resolve the decades-old conflict.
UAE
Yousef al-Otaiba, the United Arab Emirates' ambassador to Washington, said the plan "offers an important starting point for a return to negotiations within a US-led international framework".
"The only way to guarantee a lasting solution is to reach an agreement between all concerned parties," al-Otaiba said in a statement on Tuesday.
"The UAE believes that Palestinians and Israelis can achieve lasting peace and genuine coexistence with the support of the international community."
Qatar
Qatar said it welcomed efforts to broker peace but warned that was unattainable without concessions to the Palestinians.
A statement carried by state-run Qatar News Agency said the country "welcomes all efforts aiming towards a longstanding and just peace in the occupied Palestinian territories". It said Qatar "appreciates the endeavours of President Trump and the current US administration to find solutions for the Palestinian-Israeli conflict", but added that "all solutions should be consistent with international law and the relevant UN resolutions".
"All Arab states, through the Arab League, have adopted in 2002 the Arab Peace Initiative, which articulated a set of principles conducive to a just peace," the statement said.
"The State of Qatar notes in this context that peace cannot be sustainable if Palestinians' rights in their sovereign state within the 1967 borders, including East Jerusalem, and the right of return are not preserved."
Source: https://www.aljazeera.com/news/2020/01/world-reaction-trump-middle-east-plan-200128173439574.html
Not only no Arab country supports Trump's peace plan, on February 1st the Arab League stated that it unambiguously rejects it:
The Arab League foreign ministers meeting in Cairo said the plan did not meet the minimum aspirations of Palestinians, and the League would not cooperate with the US in implementing it.
The ministers affirmed Palestinian rights to create a state based on land captured and occupied by Israel in the 1967 Middle East war, with East Jerusalem as capital, the final communique said.
It's worth emphasizing that this opposition is shared by the Arab countries which are US allies:
Foreign ministers from Egypt, Saudi Arabia and Jordan, three close US allies, as well as Iraq, Lebanon and others, said there could be no peace without recognising Palestinian rights to establish a state within the pre-1967 territories.
A few more details from The Times of Israel... which emphasized some disagreement in the Arab camp, but also some apparent backtracking leading to the consensus statement.
“It is important … to come out with a constructive stance, a realistic stance and a positive strategy that goes beyond just condemnation,” UAE Minister of State for Foreign Affairs Anwar Gargash said, according to Riyadh-based Arab News.
Nonetheless, the Arab League, a pan-Arab bloc of 22 countries, said in its statement Saturday that it “rejects the US-Israeli ‘deal of the century’ considering that it does not meet the minimum rights and aspirations of Palestinian people.” Arab leaders also vowed “not to… cooperate with the US administration to implement this plan.” [...]
An unnamed Arab diplomat told the Haaretz daily that the US had not fully briefed envoys from Bahrain, the UAE and Oman on the details of the plan before they agreed to attend its unveiling ceremony on Tuesday.
The disappointment with the details of the plan led the three countries to join the other Arab League member states in voting Saturday to reject the plan, he said.
