First, "best" is a relative term.
Second, you cannot simply upgrade without asking your motherboard/system manufacturer what processors are supported by THEIR BIOS. Once you do that, you can select from those. An upgrade processor must be socket, chipset, and bios compatible. And, that is only if the existing processor is NOT soldered to the motherboard or a daughter card. If a daughter card, you must contact the motherboard/system manufacturer.
Third, there are many, many processors to choose from. Often, the motherboard/system manufacturer will have a list of supported processors. Look for that on their website.
Last, to give you an idea, look here for SOCKET compatibility. You still need to ask the motherboard manufacturer which they support:
Doc (not an Intel employee or contractor)