i would say any of them. they are all incredible shavers. if i had to pick one, i'd say the "G" as they seem to be the most plentiful and you can get a nice clean one for around $10-$15. you can find 'em here and on the 'bay.
injectors changed everything for me. my very first shave with one i knew i'd just found what i'd been missing with de shaving.
g'luck to you!
I like my G1 and my L1. If you can get one, though, you might want to get the I2 with the "Hydro-Magic" lever which makes it very easy to rinse out. All of these are easy to shave with, and I'd have trouble picking between them on that basis.
Whatever you get, take your time and be picky, and you should be able to find one in great condition for not too much money.
I think the J series is a good first choice.
I've used an A, E, and J, and the J is probably the smoothest and mildest of these.
The Type E (butterscotch handle) is the most agressive of the three, for me. Too much so for comfort, in fact.