A bit late, but anyway:
If you are set on finding any prime, then SGS (for CPU) and GFN-15 (for GPU) are your best bet. You have a 50% chance of finding a prime in SGS within 14000 tasks and my old i3 managed 4 tasks per core and hour. It's not large enough for entry in T5K, but still a large prime you can use to impress girls (or boys).
If you want entry into T5K, PPSE (CPU) and GFN-16 (GPU) are the best choice. Lower probability of course. On average it takes 2.3 * [number_of_digits] attempts to find a prime. The chances are increased greatly by sieving, as you only test pre-selected numbers. So you can say roughly 0.046 * number_of_digits tasks are required on average.
If you want a mega prime, i.e. one with more than 1 000 000 digits, try PPS-mega (CPU) and GFN17-mega (GPU). The GFN tasks are very fast on GPUs, so I guess it's the best way to find a mega prime.
All the other subprojects are only interesting if you want to find special types of primes.
Primes: 1281979 & 12+8+1979 & 1+2+8+1+9+7+9 & 1^2+2^2+8^2+1^2+9^2+7^2+9^2 & 12*8+19*79 & 12^8-1979 & 1281979 + 4 (cousin prime)