I agree with Casey — the one you’re motivated to learn. None of them are related to Tagalog or English. Both Japanese and Chinese will involve learning characters, whereas Korean and Thai use alphabets. Thai and Chinese have lexical tone — not a big deal if you work on it, but in practice lots of students find it daunting. On the other hand, Chinese has the most native speakers, the most resources, and probably the most usefulness. But you really need to choose what you love, because you’re going to be studying for a long time. 

I've always been told that Chinese is the easiest to learn. Honestly, if you're studying Japanese you'll have to learn Kanji anyways so...if you already want to learn Chinese I would say go for that. However, Japanese and Korean are very similar grammatically so if you learn Japanese you can easily pick up Korean and vice-versa.