HG: go for HG if you prefer 1/144 since it is simple, cheap, and the best among any other 1/144 scale (especially the new ones).
MG: if you prefer 1/100 i'd say go for MG, the parts are bigger so it's easier to do details and connects parts. but they are kinda expensive, especially the good ones (usually the newest, with better proportion and all) and MGs are usually popular amongs pro modeller.
PG: i don't know if people still buy PG today. kitbashing PG might not be a good idea since they're expensive and most of them are pretty outdated on quality. MG is a better choice for a bigger scale.
RG: people might want to consider sharpening their skill first before doing any serious modification on a RG, but if you simply want to swap the parts i'd say go on since RGs are interchangeable for as long as they use the same basic frame.
of them all i mostly recommends HG. and i recommend using the newest HGs such as; - HG build fighter lines - HG all gundam project - HG 00 (second season and up) - HG AGE - and some new HGUC
....for your "base kit", they all have good joints. some kits are part-swap-able. it is the most popular grade among kitbashers, and i personally prefer HG over the others, or if you're lucky with money and like a bigger size, i recommend the MG. but then again, just go with what you like the most. oh, and you might want to google up reviews for certain gunpla you're interested in.