Well, not sure that there is a 'best' external HD? But what ports are available on your MBP - I have the spring 2013 model w/ USB 3.0 & Thunderbolt ports. I bought two (also have an iMac) Seagate USB 3.0 1 TB drives (under $100 each) to run Time Machine on my computers. Western Digital is also a recommendation and about the same price. Now these are platter HDs and not SSD, which would be quite expensive!
I would suggest looking on Amazon just to see 'what' is available and for review of the comments. More recently I bought an external HD from
OWC (Other World Computing) which is often referenced in this forum - a 1 TB model for about $120 - partitioned and used to 'clone' my 2 computers w/ CCC (Carbon Copy Cloner). Of course, there are plenty of other online sites, like New Egg, etc. - I'm sure others will 'chime in' if not already as I'm typing - good luck and please come back w/ further questions, as needed. Dave