What is the best and worst cpu replacement for my i5-3230M rPGA988B?

People don't normally upgrade laptop CPUs… That first step would be to determine if the CPU is soldered on to the motherboard. If so, you can't upgrade it anyway.

Laptops are not designed for either CPU and graphics upgrading, and of course, you void all warranties and it is a very difficult disassembly and re-assembly, plus finding laptop CPUs. You will need isopropyl alcohol in high % for cleaning and then new thermal paste.

There are several things needed for a CPU upgrade.
1) Soldered vs socketed
2) Match to the socket
3) Match to the core logic (Chipset, though now only one IC)
4) Match to wattage for power and heat removal
5) Match to the system Bios to recognize the CPU. This is a secret list in the motherboard code that starts before the system registry and Windows and CPUs in laptops are often a list of acceptable by the laptop brand, so best is finding a similar laptop with a better CPU, hopefully as a common motherboard.

Let us start with step 1, Soldered vs socketed.
i5-3230M was/is sold by Intel in either version.
http://www.cpu-world.com/CPUs/Core_i5/Intel-Core%20i5-3230M%20(BGA)%20Mobile%20processor.html
http://www.cpu-world.com/CPUs/Core_i5/Intel-Core%20i5-3230M%20(PGA)%20Mobile%20processor.html

If it is CPU part number AW8063801208001, various 2nd and 3rd generation core i5 meet the PGA socket.
It will not affect gaming to change the CPU as you are at the highest integrated graphics HD 4000, allowed by the Core Logic and socket, and core logic I believe will limit you to 3rd generation i5 and i7 Mobility in G2 PGA package.

There are no cheap cpus.
These are the better-than or equal to
i5-3230M @ 2.60GHz 3953 35W
i5-3320M @ 2.60GHz 4012 35W
i5-3340M @ 2.70GHz 4159 35W
i5-3360M @ 2.80GHz 4310 35W
i5-3380M @ 2.90GHz 4220 35W
i7-3520M @ 2.90GHz 4552 35W
i7-3540M @ 3.00GHz 4735 35W
i7-3610QM @ 2.30GHz 7488 45W
i7-3612QM @ 2.10GHz 6839 35W
i7-3632QM @ 2.20GHz 6976 35W
i7-3720QM @ 2.60GHz 8308 45W
i7-3740QM @ 2.70GHz 8432 45W
i7-3820QM @ 2.70GHz 8555 45W
i7-3840QM @ 2.80GHz 9003 45W
i7-3920XM @ 2.90GHz 9202 55W
i7-3940XM @ 3.00GHz 9203 55W
CPU, passmark non-gaming speed, and wattage.

You still would need to match to the core logic, decide about wattage, and figure out what your laptop will boot up with or get rejected as a non-authorized cpu.
It is expensive, usually bought on ebay used, difficult to replace, and is a risk of working that you try to minimize.
If you have Lenovo ThinkPad T530 model 23594LU
Lenovo offers the hardware maintenance manual
http://download.lenovo.com/...474_02.pdf
and service parts list
http://download.lenovo.com/...131007.pdf
These have potential of working:
04W4139 N CPU Intel Core i7-3520M
04W6814 N CPUi7-3610QM PGA
04X0323 N CPU ASM i7-3630QM PGA
04W6812 N CPU i7-3720QM PGA
04X049 N CPU ASM i7-3740QM PGA
04W6813 N CPU i7-3820QM PGA
04X050 N CPU ASM i7-3840QM PGA
04W6815 N CPU i7-3920XM PGA
if they used a common bios and cpu cooler and motherboards or similar motherboards to:
04Y1313 N Dual core i5-3230M

If your system is working, it does not pay to upgrade to anything in dual core, so you would be looking for used quad cores in PGA as shown above, and still take a chance.

However, you would likely have a different brand and model, and try to hunt down what is on their service part list.