Magpul Green Label or STAR (both made by the same company, the magpul green label is their "economic" line and the OEM is STAR).
68 rnd. or 75 rnd. mid caps are probably the best.
As for speedloaders, there are the $9 "pistol mag" ones that are pretty good (TM makes one and Capital Airsoft sells them as well) or there's the TM XL 400 rnd. M4 magazine shaped one for like $20 USD here's a link
KA makes one as well and it's $12 USD.
Then there's always the classic rod + tube loader that's if you want to go old school. They tend to break at the ends though and I highly suggest if you're going to go that route to use an old inner barrel (ie. if you upgrade to a TBB you can use the old barrel and the rod to force the BB's into the mags).
A cool trick is to fill the rod with 30 BB's and cut it at the end so that you know how many BB's you have in the mag instead of counting it, that way you can easily fill a mag with a "low cap" number of rounds if you need to.
I'm a really big proponent of midcaps because of their versatility you can fill them with 30 rounds if you want or 68 rounds (or more).
Also don't get the "electric BB loaders" I've heard bad things about them and it's supposedly much better to get a manual one.