Easy; the Gloucestershire Warwickshire Railway (which I do) because it has a proven track record, despite its track being broken in 2 places over the last couple of yearsdue to landslips! I have also donated to the GWR's Broadway extension which is an excellent example of progress attracting more funding which in turn gives more progress etc. I donate to the LMS Patriot Project and the 82045 project because both are projects which have talented, experienced people running them and they are building locomotives that, in the case of the Patriot, fills a real gap in the LMS locomotive story and in the case of the 82xxx, provides a really useful locomotive for heritage lines that would not look out of place on the NYMR, Swanage, SVR or the GWSR. I would, if I could afford it, also donate to the 4253 loco group on the KESR because they are an excellent example of an established centre taking on a Barry wreck with the benefit of several others having already been restored. Their project plan, website and amazing progress so far says it all.
As regards infrastructure, heritage lines really cannot afford not to have secure covered accommodation for steam, diesel and rolling stock. Without this, we are just putting off eventual extinction or theft. So something like the new diesel shed at Toddington, the Mid Norfolk Railway shed appeal and similar projects would get my money if I had any left.
Then of course there's the nostalgia viewpoint. I support the S&DR Midsomer Norton project financially as I just think they are doing a brilliant job recreating a bit of the proper S&D. Several heritage railways hold S&D themed events but there is only one place to hold a proper one (well 2 if you include the excellent work being done at Shillingstone) so Midsomer also gets my support. It is also doing things in a very steady, measured way and has the support of the local authority-very important.