We can all admit that the Genesis and even the Master System, were not exactly known for their RPGs. There are certainly some important ones like Ys for the Master System, and we cannot forget about the Shining Force-series, but otherwise, it is hard to remember many for the 8-bit system, and I would be surprised if titles like Beggar Prince, Sword of Vermillion or Legend of Wukong would ring a bell. However, while NES and SNES had access to plenty of RPGs, with the most noticeable being those by Square and Enix, Sega had the clever idea to make a contrast to these medieval/steampunk RPGs and instead go with a Sci-Fi setup. This is the mindset for Phantasy Star, which spun multiple titles in their series, including the online-series that I might never review because of the danger in dedicating yourself to an MMORPG.
The original team that created the Phantasy Star-series, was a humble one consisting of only 10 people, including Yuji Naka as the main-programmer. It was quite the interesting landmark that pushed the Master System to its limit with 3D-dungeons, mature story, and one of the few noticeable titles to feature a female lead. After this, the later entries were only on the Genesis, with each title trying something slightly different, until finding their footing with the fourth game. Regardless, the series was definitely noticeable, as it was a first-party title and one of the few turn-based RPGs for the system.
Phantasies over fantasies

While the series had its ups and downs, I would like to commend it for not censoring much, outside of some oddities. It still kept the adult humour, and was graphic for its time with murders happening to either playable characters or those close to them, making situations feel dire on a personal level. While many series had to be Americanised, Phantasy Star changed little to nothing in order to keep the unique art-style, humour, and even ideas of honour that is very familiar to the Japanese culture.
Phantasy Star clearly wanted to give the player a unique universe to get lost in, as well as an alternative to the traditional RPG with medieval setting. I love how the games at least tried different elements which each instalment, be it the 3D-dungeons in the first title, auto-fights in the second game, alternate storylines in the third, or the combo-attacks in the fourth. This showcased that each title tried their best in order to stand out more than just through visuals and while the series had its struggles, it is hard to not smile at their ambitious attempts. Also, I am at least happy to say that the Master System and Genesis got at least one fantastic RPG each thanks to this series.
Huston, we have a problem

Sadly, there is a reason I say only one each. While the first and fourth games were fantastic landmarks that still holds up, Phantasy Star had a huge trouble moving from 8-bit to 16-bit. You can already see that when the original game was 4 megabites big, while the second was just 2 bites bigger, but had less in it. Both 2 and 3 had terrible grinding, uncomfortable combat, and were slogs to get through. Maybe they were necessary evils to get the fourth title, but it does not change the fact they were underwhelming and their journeys were filled with forced padding. As for the series as a whole, it can become inconsistent in providing a constant idea for its mechanics or even universe, due to different teams and developers joining in. While it all would eventually add up to a fantastic conclusion, it can come off as a mess in the middle-part.
Multimedia galaxy
Despite that the Phantasy Star RPG-series ended with a nice conclusion on the Genesis, an Online-game was demanded by Isao Okawa for the Dreamcast, despite that no-one wanted to do the project. Eventually, Sonic Team with Yuji Naka as the leader, took on the job right after finishing up Sonic Adventure, and experimented with ChuChu Rocket before looking at other titles like Diablo, Ultima Online and EverQuest for ideas. However, Diablo were their favourite take and the safest in their minds, and thus we got Phantasy Star Online, which would spawn its own line of Phantasy Star titles.

I have only some memories of seeing the first entry on the Dreamcast, as I was more familiar with the Gamecube version with episodes one and two included, but since online was still a new thing, I never got to play much of it. There are also the Third Episode for the Gamecube exclusively, and the Universe for the 360, but as of now, I have yet to actually play them, and have barely seen them in action. Not to mention, due to the amount of hacking and how new Online-gaming still was, I think the frustration would be more than unbearable to me.
Then we have the Phantasy Stars titles on portable systems, such as the appropriately titled Phantasy Star Portable 1 and 2 for the PSP, Zero for the DS and Nova for PSVita. I have no experience with the titles for the Sony-systems despite owning them, and I honestly do not know what to think of Zero. However, Japan got in 2018 Idola Phantasy Star Saga for the phone-devises, and even better: Phantasy Star Online 2, the sequel released years after Universe for the PS4 in 2016, and later for the Switch 2018, both exclusively to Japan.

To be honest, since online-gaming is a bigger thing these days, I believe it is a good time to see a return of the Phantasy Star Online-universe and get more people involved, especially for a console that does not contain many MMO-options. As for which title to go by in the original quadrilogy, there are really only one for both the Master System and the Genesis to consider. The original is the more ambitious and unique, while the fourth game hits every right note, but both are essential for enthusiasts of RPG or newcomers. Especially when they can be as cheap as two bottles of cola.