As a prospective Chemistry undergrad student for Spetember, I may be slightly biased, but I'd recommend Chemistry.
If you're unsure what you want to go on to do, I'd say any of the core sciences or maths would be the subjects to go for, as Unis will like the academic rigor they offer, and the transferable skills like problem solving and analysis are useful in any walk of life.
Yes, the subject is difficult, but then A-levels are meant to be another step up, so that's the case with most subjects. Plus if you have A* from your Chemistry exams, and keep up to date with the workload and revise well, you should be able to cope with it. I did Biology and Chemistry at AS (as well as Maths and German), and got along well. Biology was the hardest one for me actually, as it has more fact learning than Chemistry (at least that's the case on my syllabus, OCR).
Buuuuuuuuuuuuuuuuuuuuuuuuuut... The most important thing is, chose the subject you enjoy. If you think you'd enjoy Geography more, go for it. I didn't even take it for GCSE, so I don't know much about it myself, but as with almost all A-levels, there's going to be A LOT of work needed to get top grades, and it's muchg easier to put in that work for a subject you really enjoy.
Hope this has helped, and not just confused you further.