I would say it's a toss up between EPCOT and Magic Kingdom for a one day solo trip.
I will note, you can get a Hopper Ticket now and do both of these parks on that same day. I can explain that below.
All day EPCOT area:
When thinking of spending a day at EPCOT, I personally would include everything around EPCOT - not just the park it self. Of course you have World Showcase with the 11 countries to checkout food, small bites and big bites, amazing drinks. Given your date of early March, starting March 3rd EPCOT's "Taste of Epcot International Flower & Garden Festival" starts up (more info here:
https://disneyworld.disney.go.com/event ... -festival/). That enhances EPCOT's already amazing food and drink offerings, as well as festival entertainment. For rides, you have Soarin, Test Track, Frozen, Spaceship Earth, Living With the Land, Nemo, Mission Space and MAYBE Disney will open the new Ratatouille ride in France by then too. THEN you can take the new Skyliner from the International Gateway area near France to the new Riviera resort that also has amazing food and drink options, and an overall amazing atmosphere. Some people don't like it and think it's a bit bland, but from everything I've seen so far, I think it looks incredible and is FILLED with that Disney magic detail. Of course you also have the Boardwalk area resorts that you can take a Friendship boat too from the International Gateway too: Boardwalk, Beach Club and Yacht Club resorts for their own fun and unique offerings.
All day Magic Kingdom area:
The Magic Kingdom area is by far my favorite spot on the entire property. You could start with breakfast at Polynesian's Kona Cafe (make reservation) and then take a boat to the park for all the great rides - classic ones and new ones. Without FastPasses it may be challenging to get a large number of rides in, but from what I'm seeing with the crowds lately on live streams, it can be anywhere from 10 to 30 minutes or for most which isn't bad at all. Also with the first week of March being your day, that is after most school Feb breaks, and if that day you plan on going is a week day, even better for you! For dinner, I would recommend hitting up the Contemporary resort's California Grill. You will most likely need a reservation here. MK has been closing around 6, 8 or 9pm lately. Right now for all of the first week of March it's 6pm. But these times have changed almost every time Disney releases "shell hours" foreach park so you know the earliest closing time they plan for each day - they only ever will go up if they do change them. This will be a bit of a splurge, but you won't forget this experience. At any time during your meal you can step out on the observatory deck overlooking Seven Seas Lagoon, Magic Kingdom and the sun setting over the Grand Floridian in the background - bring your camera!! If you didn't want to splurge on dinner but still have a great time around the MK area, I would recommend taking a boat from MK to the Wilderness Lodge. They have great dining options, but our favorite is Geyser Point near the Copper Creek villas. Great outdoors (with heaters) lounge style dining option with incredible food and drinks. Great prices too relative to other locations around the property.
Park Hopper plan:
If you are interested at all in getting a park hopper ticket, I will note for hopper tickets right now, Disney requires you to get a ticket, and make your park reservation at the FIRST park you plan to go in the morning. Then starting at 2pm, you are free to enter any other park you want with the hopper portion of your ticket.
I think you would have 2 options here for a park hopper day: EPCOT <> Hollywood Studios or Magic Kingdom then and EPCOT.
If you wanted to do EPCOT and HWS there are easy transportation options between the two parks via the Skyliner and Friendship boats to and from each park.
If you wanted to do Magic and EPCOT, you would have to take a ferry boat from MK to the TTC and then get on a bus to EPCOT. Unfortunately there currently isn't a way to get FROM EPCOT to MK directly as the EPCOT monorail service is not running at the moment. That is okay IMO because EPCOT is usually open later than MK so you'll be gaining some time by ending your day at EPCOT.
Sorry for the large post, but planning this type of stuff is SUPER fun for me lol - especially when we can't go at the moment, and probably not for the rest of the year. If you any questions about this stuff, please don't hesitate! I'll give my best shot at helping you try and plan the best day you can.