The languages with the most loanwords from English: There are several candidates, depending on how you define "loanword" (which is not quite clear in the case of creoles), "language" (as opposed to a dialect), and how you define "South-East Asia" (does Papua New Guinea belong to it?). In general, there are quite a few South East Asian countries where English is used a lot, but a good country to look for the language most influenced by English is Singapore. In most other countries, such as Malaysia and Philippines, the use of English is in decline. This is not the case in Singapore.
Singlish (Colloquial Singapore English) is one of several South-East Asian English-based creoles and uses a huge number of English-derived words. Since it's a creole, some people may not agree to call them loanwords. Moreover, even though it is widely used, few people have it as their first language (but there are some). It is also possible to view Singlish as a dialect of English. This issue is very hard to resolve, as there is a whole continuum between the basilect and the acrolect.
Tok Pisin is without doubt a language, separate from English and with many native speakers. But it is an English-based creole, too, so we still have the problem with defining what a "loanword" is. There is definitely a very large number of English-derived words in Tok Pisin. The language is spoken in Papua New Guinea, so you need to consider how you define South-East Asia: some definitions may include New Guinea and some may not.
If you want to exclude English-based creoles, it's still likely that the language we're looking for is from Singapore, due to the use and status of English there. A good candidate is Singdarin (Colloquial Singaporean Mandarin), which is a language clearly separate from English, has a large number of English-derived words and there is no controversy that we can call them loanwords.
The question about the language with the fewest loanwords from English is much easier to answer. It is Sentinelese, spoken on the North Sentinel Island in the Great Andaman archipelago. It is the language of the few remaining uncontacted tribes, and therefore, even though nobody outside the island knows the language, we can be quite sure that the number of loanwords from English is 0.