Welcome!
You've made a great choice by choosing a 309
and you won't regret it!
On finding the best model, well it all depends on your needs and how you define best. Sport (++sensations but ++consumption)? or comfort (and correct consumption)? carburator (reactive when hitting gaz) or fuel injection (a little less reactive but fuel efficient)? 5 doors or 3 doors? For some the GTI 16 and the GTI 8 are the top of the line, but they are expensive and hard to find in good shape.
In my opinion a great model would be a well conserved/equiped (power steering, power windows and power door locks) 309 GR,SR,SX,XS,GT with a petrol carburated 1.4L engine (no power steering for this engine) on to 1.6L or 1.9L carburator or inj engines. The 1.1 and 1.3 Simca engines are a bit underpowered, but indestructible!
I think the last models from 1992 and 1993 were better built and had a lot of errors corrected, but no carburator available if i'm not wrong. Take a look here for some special versions of the 309, some of them where exclusivly built for the British market.
http://aebergon.perso.neuf.fr/Peugeot/page_Peugeot.htm
Wait for some other advice though
I think "Alargeau" is the one who knows the best the 309 english market around here and its engine versions, he'll have some valuable information