I am by no means a expert and all you guys have alot more experience and pinballs than myself but I tend to go for particular themed pinball's - water / beach. As I know that at I can live with the looks and coupled with being a good player - it's a great package. Maybe look back on what you did for fun in the day and then find a pinball with that theme if there is one.
Today I picked up my Bally Skateball which is a 1980's pinball (my only early SS). People either like it or hate the look but to me it's different and retro late 70's so I like it. It also has a very similar layout to the Williams Flash which is interesting. Only had it today and loving it - sounds are basic and not annoying and the game play is fun and challenging and love the fact you can see the ball all around the playfield.
I vote towards a Fathom, great game and awesome looker - one I definitely would love to own one day.
