It depends on where the Test is being played. In India, it's almost always 2nd day is best for batting, while in AUS, Day two wicket often gets faster. Also, it depends on the match condition - batsmen of team with nose ahead will approach the day differently compared to team behind.
In general, in a standard contest, in terms of batting difficulty, often it'll be like (Most to least)
India/BD/SRL
Day 5> Day 4/Day 1> Day 3> Day 2
These days India is producing fast, pacers' track. But, on a typical burnt South Asian track, it's toughest to bat on day 5, day 1 & 4 are parallel, because by day 4 it starts to turn while first two hours of Day 1 is always challenging in South Asia, where Test cricket is played in winter
ENG
Day 1> Day 2/Day 5> Day 4> Day 3
It's tough in all 3 sessions of Day 1, and first session of 2nd, while day 5 often brings spinners. Typical English wicket is best for batting from Day 2 afternoon to day 4 tea
AUS/SAF
Day 5> Day 2> Day 1> Day 4> Day 3
On those hard, scratchy wickets, if the track cracks a bit, those AUS wickets are the worst on Day 5. Day 2 is slightly tougher than day 1 because it gets faster & bouncier in Day 2.
NZ
Day 1> Day 2> Day 3/Day 5> Day 4
The wickets in NZ are so typical - it helps seemers on first 4 sessions, then gradually flattens out. Day 5 offers a little more help to Day 4 because of the 4th innings pressure.
WIN is too inconsistent these days and their different venues offered completely different experience. If I take Barbados as the representative of WIN, in olden days
Day 1/Day 5> Day 2> Day 4> Day 3
It was like very typical English green wickets, with only difference is that by Day 5, spinners also come into play in WIN, much more than UK. And, cracks open up like AUS tracks. Day 3 & 4 are probably the best day for batting.
Personally, I like AUS wickets most because it offers something for everyone and all 22 players can come into equation over 5 days. In an even contest, Captains probably most confused after winning toss in AUS - bat first, you might go to 1st Tea at 141/7; bowl first - you might face target of 343 at 4th Tea.
I put it in other way - which venue gives the highest value of winning toss (In an even contest - last time one toss was won by both Captains of PAK & AUS, Aussies won both Test by innings....)
1. IND/BD/SRL - win toss & bat first, you got at least 100 runs handicap for a head start
2. NZ - win toss & bowl first, you have to play really poor not to win from there on
3. ENG/WIN - you can bowl first, but make sure that you have taken at least 3 by first lunch
4. AUS/SAF - doesn't matter if you bat first or second, you have to play well and play positive, play by sessions AND MUST hold on to your catches. I have seen Captains looking fool at stumps of Day 1 after taking a call at toss, which that time looked perfectly logical.