B/X Dnd was the gateway game for many. It took the brilliant mess that was Original DnD, the purple prose ideas from ADnD and distilled the game down to a superb set of simple rules. Great artwork, decent layout, cheap entry price. Probably the most accessable of all the versions of DnD. This year marks it being 40 years old and I still have an original boxset, bought the pdfs on Drivethrurpg and even grabbed loads of OSR clones of the B/X system. If you want to play DnD you can't go far wrong with these rules from 1981. My Adnd campaign that ran for 6 years was actually B/X with the PHB and modules from both basic and advanced. The system could handle it and gave me thousands of hours of fun, stories and friendship. Best £10 ever spent, so getting if off here is even more bang for your bucks!

edit - to fix mobile spamming words